I'm looking for info on these myself. I've been doing a lot of research, and there seems to be very varied opinions on the quality and reliability of both the Tapco and Promag detachables. They seem to have lots of issues from failure to feed, failure to load, to ejections and jams. Most seem to come from Murica where they aren't pinned, cause they all say either it "fails to feed on the first round" or "it shoots fine until 16, then stops".
Hical sells all sorts replacement mags all pinned to 5 but they are easily the most expensive place I've seen them sold. Other places seem to have the 20 and a couple have the 5 round mags. I'm looking to mount a big rail on my Sks that will prevent me from using stripper clips so I'm going to need some mags, but don't want a big bulky magazine (that is only useful if government collapses) messing with my prone, bipod shots. So that means it's 5 or 10 round mags, but they are pretty much the same price as 20 round mags. Guess it's an uphill battle...