All depends on what you want. I managed to find a guy who could get hold of some slick Michelin tyres for £120 the set but after I e-mailed him to do the deal (I wanted 2 sets) he never replied.
If you want semi slicks then the chances of getting part warns are very slim. You are better of buying new ones and if you are buying in bulk you should be able to get a deal.
I'm probably going to go for Yoko A048R's for about £60 a corner (on 14's) or £90 a corner (on 15's) which should last me about 3/4 track days.