The trouble is alot of formulae limit the number of sets you can buy at a discounted rate. Usually just enough sets to complete all the rounds leaving you to source sets for testing etc.
If its the cut slick intermediate tread patten your after give Colway a ring. You can buy directly through them.
On a durometer there isn't much difference between their inters and a set of R888's that have been softened slightly
2008: VW1 3rd in Class
2009: VW1 3rd in Class
2011: Ginetta Challenge
2012 Ginetta GT5 Challenge/Golf GTI Championship 2014 BRSCC Production GTI Championship
2008: VW1 3rd in Class
2009: VW1 3rd in Class
2011: Ginetta Challenge
2012 Ginetta GT5 Challenge/Golf GTI Championship 2014 BRSCC Production GTI Championship
That will tell you the diameter of your current tyres and you can compare diameter against the race tyre and see the effect on your speedo reading/gearing.