You can find the prova body kit at
http://www.z1auto.com/ImpLegHome1.html
which is the only spot I know of offhand, and the C-west body kit is everywhere...
www.optauto.com is one that has it, I have several more on my favorites list at home, but i am on vacation in washington DC at my sisters. I know the company Do-Luck...
http://www.booststore.com/9001imprezakits.html ...does make a widebody kit with widebody fenders and so forth, as does HPI or something like that and Trial-R both of which you can find at
http://www.allworldmotorsports.com/...y Kits&Model=Impreza RS&Make=Subaru&Year=2000 but i have never seen any z3 fenders or anything like that if that is what you are thinking of...however you can find just the z3 inserts so you could have it set into your stock bumpers...i also have a C-west style body kit for sale if you would like to buy it, its never been put on, all pieces are intact and in good shape, it includes the front bumper, side skirts, rear bumper splats, and mesh for the grill, planning on selling it for around 525, which is teh price i purchased it for, plus shipping and handling, since its in pefect condition and the only reason teh box is even open is when i checked to make sure everything made it th rough shipping without being broken. ANYWAY, also a good site if you want to see other peoples' opinions on certain items you can go to
www.subarureview.com and these people usually list the places where they purchased these items too, so check that out as well