Try to search a specific make or model and you should get several hits. Gunsamerica auction, gunbroker.com, action arms, Impact Guns, and Buds in Lexington are just a few. Be careful and buy from a FFL retail store. Buying from individuals on-line can be scary at times.
What are you looking for? I have purchased a lot of firearms from online dealers and my favorite for new guns is impactguns. If you are looking for a good deal on a used gun, go to an auction site like gunbroker or gunsamerica.