I have this:
http://www.airsoftpost.com/product_i...ducts_id=31876
It comes with 2 little stubby ones and a longer one plus whatever is on top, I can't imagine needing more than that, but just in case. Turns out there are uncut/drilled ones on ebay that I can just get and drill and cut to my desired length and mount points, if not I can order the longest one I can find and do the same... that's if I need more, which is unlikely as far as I can see.