The factory studs are about 35mm I believe. If you go with a 15mm spacer you're going to want at least 15mm more of stud. That's 50mm and a common size I believe. 60mm is also common.
I would recommend any stud other than Nismo studs. Nismo studs are known to be complete shіt. They sheer all day long either while driving or while removing lug nuts.
Personally I love the ARP wheel studs and they can be found at Summit for a good price. H&R wheel spacers are probably fine too, as are other brands.
Need more info on ARP studs (and why Nismo sucks):
http://www.sr20-forum.com/suspension/13333-arp-extended-studs.html
Need more info on sizes and a comparison of studs (more on why Nismo sucks and what other options there are):
http://www.sr20-forum.com/suspension/10256-b13-wheel-stud-knurl-diameter-size-dimension.html