If it's an impreza then try Scoobynet, SIDC, PistonHeads, Autotrader (£££), S1 Cars, Gumtree, Yellow paper (not sure if that still exists)
Most audience on Autotrader and PH I think but if it's a specialist car then Owners forums are probably better
Have a look at the spec... I'm not getting put off by a bit of bodywork that can be changed if everything else has been looked after. Spoiler is enough to make you cry but there is money sitting in the wheels etc.
http://www.pistonheads.com/sales/906908.htm
Yeah this is the car i've been looking at but that bodykit is horrible
Standard spolier and rear bumper he still has which would make it alot more respectable.