Im looking for an installation kit to install a head unit in place of the in dash 6 cd changer that came with my 2003 wrx. I went to the dealership to see if they had any extra "spacer" storage compartments that come in the 2.5 rs but they insisted that it wouldnt fit the wrx. They also insisted that the 2002 and 2003 molding around the CD Changer was different from eachother...but by my inspection it looks to be the same.

Any single-DIN pocket should fit. If you want an OEM one the RS pocket should work, there is also one available for a Legacy that has a door on it. Some people have actually gotten them free from the dealer just by asking if they had any lying around from upgrades or installations, but it doesn't sound like your dealer is one of those types.
Also car stereo dealers and stores will have universal pockets that will work as well. When I bought my HU from Crutchfield they included a pocket at no charge, along with the harness adapter.

Like he said, they make dash kits for pretty much any car anywhere. I know for a fact you can get a dask kit for the rex and if you order youre HU of crutchfield it'll come with the dash kit and an adapter harness.

You can order the pocket for the MY02 RS and that pocket will fit into a single DIN position just fine. As for the harness, it should be pretty easy to find.