Actually, I feed my rabbits straight up Timothy hay. Same stuff the horses get. Alfalfa is good for growing rabbits, but it's got too much calcium for adult rabbits if it's fed as their staple diet and can cause bladder stones. At the wildlife center I work at during the summers the buns there get Timothy, too. So think about switching it up - better for them and WAY cheaper, just grab a square bale and it'll last FOREVER!
Horse pellets should be fine as long as they're high enough in fiber/protein and whatnot. Buns have sensitive needs and their pellets are pretty formulated, but it's entirely possible horse pellets could provide what they need (I've honestly never looked, but so long as they're timothy based pellets should be good!)
Here's a little info about it: Is alfalfa hay for rabbits okay? - Small Pet Select