Below are some recommendations from community gardeners, past and present!
A gardening cooperative with subsections for seeds, flower bulbs, trees, edible bulbing plants, and garden supplies. Based in Maine, they specialize in cold-tolerant selections for the Northeastern climate; while Pennsylvania's climate isn't as rigorous as Maine's, Fedco offers varieties that early- and late-season gardeners here may find useful. Their prices are sometimes much cheaper than other sites due to their status as a co-op.
Baker Creek Heirloom Seeds
A large collection of heirloom vegetable and fruit seeds established by Jere Gettle (you may have seen him on 'Oprah'). Extensive online listings include some very rare and usual selections found in few other places. Prices tend to run a bit higher than some other vendors, though.