Remsport in Mass has some nice stuff. Match barrels and satinless slides, ported etc.
And you can buy the uppers, parts etc al day long. Many online places will ship those as its just parts and none of it is considered the gun. But a complete upper new goes for the same price almost of a used glock here. (please dont post names if you find one- std disclaimer here)
Remember that the slide is the expensive part on a glock. The frame is just a bunch of plastic. So for a new $600 glock, the slide is about $400 and the frame is about $200
You can get used for $429 a glock 22 which has a full size frame, and you can change the barrel out between 9mm and .40, I think even .357sig. without changing the slide.