Genuine aftermarket kits fit well.
There's a whole bunch of body kits available for the s2000. However most "bodykits" look shit on the s2000 the stock body dosen't really need much improvement.
There isnt really a general website on "bodykits"

Best bet is to go out and buy a HyperRev magazine they will have most JDM brand bodykits available. Shipping time from japan for most of these kits will be about 2 months by sea + production time.

Or google S2000 Bodykits and find which one you like from there.

Unless you want to go the local replica option which the 2 most common would be copies of the Cwest kit and Veilside kit.