You can get them from Midway or Brownells.
Two different companies make them, McCann or Schuster. They have different designs from each other. I've never used them, but to me it looks like the Schuster is more adjustable (which would be nice, since you can "tune" it more precisely than the McCann, while the McCann is more repeatable (which would be nice if changing ammo, and then back to another style you have shot before.) Someone who has used them can comment as to if my thoughts are more or less true, and can share their experiences with them. I have read that guys can increase precision with surplus ammo, but obviously I don't know if that is true or not. IMO, they would be great if you wanted to hunt with an M1 (and don't handload.)
I've never shot the Hornady ammo, but I can't imagine that it sucks. I'm not sure if it is worth the price though. (It hasn't been for me so far, but I also handload for my M1 when I have time.) Again, others who have used it can comment on it, and can actually speak from experience.