I agree with most of the others - boots, breeches, half-chaps, and helmets are equestrian brands. Beyond that (shirts, jackets, etc.), anything goes.
I did used to ride in heavier-duty leggings (like Mane, I had a pair that had nice deep pockets on both legs, which was nice), and they worked well enough. But they aren't made to hold up like breeches are, and I found they got holes and stretched much more quickly than even cheap ($30-$40) breeches.
Riding: The art of keeping a horse between you and the ground.