Depending on what you are looking for there are many shops in the area... (Bicycle warehouse is mostly family bikes)

Jenson USA has their storefront shop up by the 91/15 interchange, off of Countryclub pkwy/sefras Road, and their warehouse is by the 215/91fwy. The warehouse has a walk up counter and a small show room.

and the guys at Stage2 bicycles in Murrietta are pretty cool. (They carry more high end stuff)

if you're going down towards San Diego, there's Bike Bling and the Trek Superstore in Escondido.