It's not needed for healthy rabbits though, and some might chew it. I like to give them grass mats to lay on, and my hotot girls also get several flour sack towels laid on top of each other. They love them and since they aren't fluffy they don't chew on them like they would other towels. Of course, they would be a midnight snack for Mocha and Loki, so they don't get them!
Edit: Aside from internet stores, you also might be able to find vet bed type stuff at pet stores in the dog section.
They are also great for your bun to sleep on and for soft flooring if they have sore hocks, and unless your bun is very determined they shouldn't be able to chew it, mine haven't managed yet!