I got it from T-tiger or something; it's a Hong Kong based site.
It isn't for sale there anymore unfortunately, I was able to get the last one before magpul got them to removed it. They might actually have a secret catalog so you can try to email them a special order for it. I have seen several other chinese sites with element magpul parts which were all removed, try ordering from there as well.
Try checking out airsoft-club.com
www.pro--t.com
acmgear.com
and other chinese sites, just email them to see if they have them, though the price would probably be higher.