http://davesgarden.com/products/ps/ Dave's Garden has a feature like that. I also use their "Garden Watchdog" feature a LOT--it allows customers to rate businesses and give reviews.
http://bazaar.gardenweb.com/directory/ Garden Web has a listing by broader category (ex: bamboo). They also have forums on specific topics where other knowledgeable gardeners could probably assist you. (Ex: check the "edible landscaping" forum for help on locating edible plants; the "fig" forum for help on locating a specific variety of fig.)
But the most "niche" kinds of plants are still going to be more difficult to find.