Also you could buy 10/20's, or 10/30's already blocked and sealed, could even have them instal an ultimate bullet button tool.
I picked up 10/20 (pmags) with the UBBT installed for $18 (this was in may 2012).
Try caleagalmags.com, or exilemachine.com
Pmags have been scarce lately and a bit pricey too, not to sure if you'll be able to find them for the same price.
Another option is to get a usgi mag install a magpul rangerplate and install the UBBT into the ranger plate instead of the mag.
Just realized the UBBT might not be popular on this site. It is a little knob (for lack of better description) that is installed into the lower section of the magazine (for or aft) or installed into the ranger plate. The tool is rendered useless while the magazine is inserted into the rifle, so it makes it legal to use. It only comes in handy if you have multiple magazines with the UBBT, using the next mag as a tool to unlock the spent mag inserted into your rifle.
if you look closely at any picture I have uploaded of my rifle you will see said knob at the lower front part of my magazine.