Daves Fastglass Resin from any good Halfords. not too expensive and the big 1l tins last enough for a suit if just fibreglassing.. you'll need more if you're mixing it with body filler to make the 'rhondo' casting stuffs.
more than likely, I managed the torso and helmet of my HEV suit with 2 cans and I really... really re-enforced the Torso section about 3-4 layers of Fibreglass Strand Mat.
Depends what sort of resin you want.
If you are fibreglassing then I would use East coast fibreglass. Good prices and the resin is good. Look for the polyester resin.