I have noticed a couple of posts mentioning that rubber feet improved the sound slightly. Is this accurate for many of you? If so, where can you get them?

do you mean putting rubber feet on the bottom of a floorstander or sub (or any speaker that is sitting directly on the floor)?
In that case, what rubber feet will do is keep the actual box off the ground and that way when it vibrates it won't sound like it's bouncing on the floor.

If you mean putting rubber feet on bookshelves that will sit on a desk or other furniture, then the same applies. If you are putting them on stands, the stands usually come with rubber stickies you can put on the top plate the speakers will sit on.