Look at a Farmer's distributor. Like a Farmer's Co-op or something like that.

I find bagged alfalfa at my local nursery, but it's a bit expensive for a larger number of roses. Finally purchased a 50 lb bag of pellets at a feed store (horse food). It's way too much, but was only $11. You have to be careful there is nothing else but alfalfa in the pellets. They don't break down as fast as the bagged stuff from the nursery which is like a powder almost.

Check to make sure that the alfalfa pellets do not contain salt or other additives that may cause problems, many pellets do contain additives when they are labeled for horse feed. I buy plain alfalfa meal, a powder with no additives, at a farm supply feed mill for about $11 per 50# bag.