Make your own. Check the gallery or PM Daes for the instuctions on how to make it. He said the hardest part is getting the brass tube to line up so the bbs feed ok. It's fairly cheap and his holds 4000BBs with an insulated box so you can't hear the rattle.
http://www.airsoftcanada.com/gallery...&searchid=1590
http://www.airsoftcanada.com/gallery...&searchid=1590
http://www.airsoftcanada.com/gallery...&searchid=1590
http://www.airsoftcanada.com/gallery...&searchid=1590
Daes is the man for making box mags.