The only food I can usually recommend that is atmost pet stores is Kaytee Timothy Complete if your rabbit is anadult. I prefer Oxbow pellets above all else, but I doubtPetco has that.
If your rabbit is younger, try finding a feed store. Thereare several very good brands of rabbit food that are marketed for showrabbits and such. Purina is the only one that comes to mindat the moment. It's far less expensive for better qualitythan most pet store rabbit pellets. $10 at a pet store willget you 10 lbs or even less. $10 at a feed store can get youmore than twice that (I think actually 50lbs?).
As a general guideline, look for plain pellets with at least 18% fiberand 16% protein or less. Don't get the stuff with the treatsin it- it's really unhealthy.