My local hobby shop, Scale Reproductions, Inc., in Louisville, KY has the most complete plastic model department I've ever seen. Brian Bunger, the proprietor, is a plastic modeler himself, and hired his old friend Marty to manage the plastic department. Marty has done a masterful job of stocking and supporting the department and it is first class. Every new kit that's reviewed in Fine Scale Modeler is in the shop at the same time. If I wasn't building my monster railroad, I would be buying lots and lots of plastic kits from them. They also have fully equipped, RC, HO/N trains, and slot car departments. It's a treasure since almost everywhere, good full-line hobby shops are going out of business.
For Internet, I've just started buying at Sprue Brothers and am impressed with what they have in stock.