It would help folks find what you are looking for if you include a link to the sound you are referencing. Not everyone here has watched GOT (or even know what GOT stands for).
I often use zapsplat when I search for sample-based sounds for my project. I searched "horn" on Zapsplat and while there are a lot of car horns
that come up, there are also large truck and/or ship horns.
If you're willing to open up an audio editor such as Audacity
you could probably layer both these sounds together, apply a little reverb to the overall mix, and then add a slow fade in to make it more like a "blown" type horn.
You could also find something like maybe a tuba or trombone and then pitch the sound down to make it more dramatic.
If you aren't picky about the sound, I'd be happy to design the sound based on the above examples. But last time I designed sounds for someone for free, I spent 2 hours of my time to help them and in the end they decided not to use anything I made for them. I don't really want a repeat of that.