Is difficult as Impreza prices are very strong atm. Patience is key and get in there when a good deal comes up. Saw this recently for £950 but it sold in a day or so!
http://www.scoobynet.com/subaru-444/1037407-modified-ltd-578-1000-wrx-v2-sti-8-block.html
Best of luck with the hunt fella