Your most economical option is to go to a feedstore and either buy a bale of hay, or, if they offer it, a few flakes from a bale of hay. Feedstores near me will sell a trash bag with a few flakes in it for about $10. At those prices it doesn't matter if your rabbit picks through it. Any grass hay is fine for adult rabbits (e.g. timothy, orchard, coastal, meadow, bermuda, oat, wheat, barley, etc.). Some of the rabbit rescues near me buy bales of hay and bag them into smaller bags that they sell to fundraise.
Another option is to get hay from someone with a horse who will let you have a few flakes at a time.