There is a Supersprint system and the Milltek system available but it all depends on your price range (and also whether you have the dual exit system that I and Leoncp11 have or just the normal one)
Mines is custom made by a local company near me and it cost me about £800 for the whole system (from the turbo back and with a sports cat, the pics can be seen in my reader's ride thread (click on the link in my signature))
Alex
p.s. Leoncp11, mines only had the one back box didn't realise that they had a different configuration on your Leon?