I don't know where they get theirs, but a better choice is any 3/4" 1/4-28 COMBO HEAD. That allows you use either phillips or STANDARD (which will be MUCH easier to find in "the field", than a hex, which nothing else on the AR uses). You can find 1/4-28x3/4" COMBO HEAD screws online for something like $15 (or less) for a 25 pack. That's what I do. I HATE Magpul screws for that reason (I have zero use for a hex screw on an AR), as I much prefer the more common method of either standard or phillips head.