you can get moleskin from cvs! it's like a thick fabric sticker that you cut to whatever size you want. it is super sticky so you put it behind your knees or on your heels. it is really difficult to get off unless you soak it off but it's soooooo cheap.
Wear thick riding socks. If you don't wear riding socks, then you breeches will ride up while your riding and will make the rubbing worse. Put on some bandaids under your socks and that will help also. I hope this helped!