Good question Rich, and there's lots of answers.
I get most of my leather from
Tandy because it's convenient and they have the kinds I need and lots of tools. There are other cheaper sources of leather, but convenience sells for me. They list horns too, but in my experience they're marginal unless you can visit a store and pick through their supply.
I get lots of hardware I need from
Track of the Wolf. Noodle around there and you'll find all sorts that's useful for bags and horns, and a lot of other accessories. Horns, too. They've also got lots of examples of finished stuff you can learn from and copy as you want.
Can't find them in my favorites file right now, but there are lots of other sources for horn and hardware, but usually not as big a selection and variety all in one place. TOW is just convenient for me.
I get most of my horns from our own cattle or from neighbors, but those days are gone for most folks. Number one, not a lot of cattle in the US grow up with horns any more, and number two, not many folks live outside of town any more. I can't say it's "easy" to get them any more, even for me. I just like the "feel" of using horn from animals I know.