dude...
i know that chris wilson had some semis for sale. but not sure what his number is...check out gtr.co.uk and search for him under members...he had some pirelli pzeros in semi for sale and they were part worn...
also look in the back of autosport as there are a few companies that can soruce the semi slicks but i cannot remember for the life of me who it is.
hope this helps, nick.