Some possible choices:

http://shop.ebay.com/i.html?_nkw=fis....c0.m270.l1313

^ Topic search.

http://cgi.ebay.com/stainless-steel-...efaultDomain_0


http://cgi.ebay.com/Spearfishing-Spe...efaultDomain_0



Fish slick makes a 6' kayak style stringer:
http://www.fishslick.com/products.html#howtouse

Ocean Kayak fish stringer kit:



You could also make your own by buying stainless steel carabiner clips, and rigging some rope, but at the prices listed on ebay, I like the diver's clip. A lot of yakkers I meet seem to prefer that one as well.