If you possess a magazine-fed firearm that you bet your life on, spare magazines should be acquired either from the company that built the firearm, or from a reputable company that supplies magazine to the firearm company.
Oh, there are a few companies out there that produce really good after-market magazines -- Chip McCormick, Wilson Combat, but in general it's best to go with OEM.
At gunshows, and in the back of dead-tree gun magazines, you will find adverts for companies that sell after-market ammo feeders. Companies like ProMag, USA Magazines and the like sell magazines which are okay to use in plinking guns, but really shouldn't come within snuggle distance of your defence firearms.
One of the more disappointing, yet amusing, experiences in my little life was slapping a USA Inc magazine into a Smith and Wesson 5900-series and getting this gorgeous brass fountain out through the ejection port before I could hit the slide release.
Factory or OEM only.
There is, however, a rather nifty use for El Cheapo magazines.
If you have a single-action revolver -- a Colt Single Action Army, Ruger Vaquero or somesuch -- the next time you're at a gunshow, pick up an El Cheapo Desert Eagle magazine in the proper calibre.
Take said magazine home, pull the baseplate and snip off a coil or two of the magazine spring before reassembling the mag.
The next time you're loading your hog-leg, hold the revolver in your left hand with the loading gate open, hold the full El Cheapo magazine in your right with the top round at the loading gate and thumb that top round into the cylinder.
Voila! Just rotate the cylinder with your left hand while thumbing rounds in with your right and you have just converted a piece-o-junk magazine into a nifty little speed-loader.
I am informed that this also works if you have a double action .22 rimfire revolver and a 25-round El Cheapo .22LR magazine.