1. Check local feed stores. Orchard grass, bermuda, and timothy are usually available there. A lot of times they'll let you fill a feed bag for like 5 bucks, you just need to ask them (dunno how many rabbits you have. We go through a bale a month between rabbits/pigs/tortoises but if you have one or 2, that's obviously overkill.) TSC will also have it in a mini bale for about 20 bucks. Don't overdo it on alfalfa, it's fine as a supplement, but as the main grass it's not ideal. too rich.
2. plant your own, If you only need a bit, might be worth the effort to start cultivating a patch.