@Tai_MT This post is what finally told me that I needed to clarify. But don't worry, 'tis not your fault. But let's go through those ideas one at a time and smack them down.

Your "Quests" idea: I want an at least slightly different way to obtain each one, as they're just scattered around the world. Your "Behind a currency" idea: So uh... where would I put the currency? Difficult to find chest? Where do I put that? Never played Illusion of Gaia, much less played it. (The forum rules say that when referring to another game as an example, you should explain it anyway, but I'm not a mod, so don't listen to me.) Your "Skill challenges: idea: No perma-fail, that's for sure. I don't have any thief or pirate party members. Your closing suggestion: I reiterate, there are a limited number of these obtainable in the game, so no randomness. Thank you for the suggestions. (How do I say that without sounding sarcastic?)
The "Slightly Different" way to obtain each of your "maximum of 8 total of this item in a game" would work as a series of Questlines, provided your Quests were varied enough. However, this sounds more like you have these items like the "Bottles" from the Zelda franchise where there are only a few, a couple are obtained in quests, some through exploration, some through clever use of abilities, etcetera. But, since I had no idea you were talking about a finite amount of items that would be consumables, and thought instead you were talking about Unique Pieces of Equipment, I offered the three things as my method of doing it. As such, each of my examples is effectively moot for what you're trying to do. Communication error on your part, I'm afraid ^_^ I'd have probably not replied if I had thought you were talking about a finite amount of consumables.
However, I will address some of the things you've mentioned here briefly (or as briefly as I can manage. I'm quite long-winded).
A "Difficult to find chest" might be hidden behind a tree, or in a passageway that isn't really apparently visible. My Diamonds are put in all manner of places, including in basic chests right out in the open. It is expected in my game that a player will find enough of them to get most of the Accessories and Relics, but only a specific search will turn up all of them. As in, you need to set out to find every last Diamond in my game if you ever plan to obtain them. The first of which is given to you as part of a Tutorial in order to teach the player the point of these things and what the person who takes them looks like. I have a few bosses that drop them as part of normal progression as well. As for yourself, you can put that currency anyplace you like. Hide it behind a mini-game or in a puzzle if you like. I'm just doing it the way that makes sense for my game. The example wasn't meant to tell you how to design your game, it was meant to give you some ideas... a sort of "launching off point" for your own game.
I did actually describe the system in Illusion of Gaia. You collect Red Gems (wherever they are... I've never collected them all) and you "turn them in" to a specific vendor for a reward. 60 gems in total, you get a reward every so many gems you've turned in, in total. You cannot skip ahead in the list and select something you want towards the end. You take each reward in turn. I explained how that worked in my post, I'm sorry you missed it. Or maybe you had a different question about the game? In any case, no reason to be hostile towards me. I was only trying to help you.
You really don't need to have a Thief or a Pirate party member. Again, my entire post was meant to inspire ideas in you, not tell you how to design your game. Designing your game is up to you. No need to rip my ideas off wholesale or shoot them down on the basis that ripping them off wholesale wouldn't work in the design of your current game. Plus, as your original post says... You're looking for ideas. Just because an idea of mine doesn't work, doesn't mean you can't find a way to make those particular ideas your own. "Skill Challenges" aren't even my idea, I first encountered them in games like New Vegas and loved the idea of them. Feel free to make those systems your own if you like how they work. Feel free to ignore them if they just don't work for your own game. You can make your skill challenges repeatable if you like. Or not include them altogether. Or, introduce them in such a way that feels interesting or unique in your own game. My ideas for Thievery and Pillaging simply come from wanting to give the player rewards for playing well, for giving their characters stats that improve their odds of nice loot (though it is wholly unnecessary).
I'm not actually sure I was offering suggestions. However, since I now better understand what you were asking, I'll provide you a few.
1. Do a little bit of world building. Why are these items so rare? What is their origin? Why would anyone from the standard populace of NPCs ever have one? How have they ever been obtained in the past? Are they even that old, or something recent? This will go a long way towards deciding how they should be distributed to the player.
2. Decide why you have so few of these. What gameplay purpose does this serve? Are you falling into the trap of the Elixirs from Final Fantasy wherein they are such a powerful and rare item that the player never uses them because they are waiting for a situation that is absolutely untenable in which to use them and it never comes? What are you trying to communicate with these consumable items that are so rare and so limited? Are they even worth obtaining in the first place? What makes them worth using in the first place?
3. After figuring out number 2 there, you should decide how much effort a player should put into obtaining them. You'll need to balance frustration with reward to avoid a "Rage Quit" of ever looking for them (a player deciding that finding them isn't worth the effort and ignores them entirely). You'll also have to balance an entire battle system around only 8 of these items existing, so you aren't going to be able to have a lot of bosses that are really powerful if these items are so rare (meaning, if a player can't revive their characters in combat without one, they're likely going to grind like crazy just to avoid party members falling in the first place, this is going to throw your balance of the game off considerably, meaning the more powerful your enemies and bosses, the more time players will spend grinding in order to avoid party members falling in combat since your consumable to restore life is so rare that only a few exist in the entire game).
Beyond that... not sure I can offer any suggestions. A lot of what you come up with is going to be heavily specific to your game and your game only. It's something you are going to probably have to solve on your own, since I don't think any of us have experience in making a game with a limited number of revival items.