They are available from Bushmaster, but hard to find on their website. It is in the Clearance section under products and is the Single-Shot Magazine Follower, Part No. SS-FOL at $13.95 each.
I bought two and installed them in two spare 30 round 5.56 mags. They roll into the mag on top of the existing follower. I haven't shot them yet, but they load hard (almost cut myself on the flanges on the top of the mag). I will get a loading device, which I think I saw on the Bushmaster website. There is a very simple one shown on
http://www.bushmaster.com/products.asp?cat=9, part number UMT for $6.50, but I thought I saw a lever type similar to the LULA also. Can't find it right now. Also, when I went to manually unload the mag, the fourth and subsequent rounds had tilted in the mag. If you have a mag with an anti tilt follower, that may help, or they may not tilt when they are actually being fired.
I will report back after my first range session.