They are made by MRI and Pro Mag.
I have no direct experience with the Pro Mag version, but there are plenty of people who claim they do not work well. Based on this, I have always stuck with the factory mags.
CDNN has factory mags for $45. That's about what I usually see them listed for. Occasionally you can find them on gunbroker for a little less.
One other thing to be aware of. The older mags did not have as much of a cut-out on the front of the magazine. I've got one of those, and a few of the newer ones. Compare the scoop at the top-front of the two mags. The one on the left looks like the bullet is going to hit it on the way out. I haven't had any problems with it, but for the same price I prefer the newer design.
Also, I haven't tried it but some people claim .44 will feed / function just fine from a .41 magazine. Sometimes you can find those at a discount.
Oddly enough, both styles of magazine are marked "7-95" over "44".
eta: I see Check-Mate is making DE mags as well. I have no experience with these. But for only $6 more (Brownells lists them at $41.50 if you get dealer pricing with a C&R or other FFL), I'll stick with factory magazines.