IMHO keep the outside stock, don't attract too much attention. that way when you are doing a few miles over the speed limit you don't look like you belong on the cover of import tuner magazine. Dont give cops another reason to pull you over if you dont have to.
Ebay is a good place to start but beware if it seems like it too good to be true it probablly is. There are a ton of car parts on ebay that are made of cheap plastic and wont hold paint if they even fit on your car.
What kinds of body kits/mods are you looking for?
13 WRX Hatch - Torqued Performance Stage 1+
Slow is Smooth > Smooth is Fast