I have to second www.squadron.com. I use them more than anybody else. I've placed a dozen orders and only once was something I ordered not in stock. They usually ship their orders the next day. Typically I get the package a week (or less) from the day I order it.
I've only placed one order with Great Models Webstore and was not at all impressed. Aside from taking forever to ship, they screwed up my order. Although they attempted to fix it, I wasn't happy with the solution.
-Jonathan