They are from a variety of sources, many of them have been around for ages from other artists (Jaime Jones, Kekai Kotaki,
Levi Peterffy, etc etc etc...) others I made myself, others I altered, and so on.
Sometime ago, someone in my youtube channel told me he had just bought a brush pack, which really struck me the wrong way since I believe they should be shared freely with everyone. I really don't see the point in selling these as you can get them anywhere online these days if you research just a bit as fortunately there are still many people sharing them for free. It's also not like you are selling some secret painting sauce, it's just a simple tool. So yeah, don't buy brushes. And don't sell them either, don't be a douche!
Also, it's been said a million trillion times, but just keep in mind that they are just a tool and won't make you magically work better. They can be helpful if you are trying to achieve a certain look in your illustrations and so on, or to give some texture, but don't focus to much on gathering a ton of brushes and using them all. Just use the ones you are most comfortable with.
The last 10 brushes in that pack are probably the ones I use the most, but 80% of the time I use the same 2 or 3 brushes, the others are just flavor.
That's it! Let me know if you have any questions!
Cheers everyone!
Jorge