I've been quietly scooping up MecGar and factory magazines on Gunbroker and Auction Arms. Nearly all of the auctions are overpriced and ridiculous for mags.
However, if you look every day or two, you can almost always find one or two that are going cheap with no reserve every month or so- then it's just a matter of keeping track of them.
I've picked up (3) MecGar stainless P220 .45 mags for $38 shipped, and (2) MecGar blued P220 .45 mags for $26 shipped. That's cheap.
I also picked up (3) factory Sig P239 9mm mags for $54 shipped, and (5)factory Springfield XD-9 15-rd 9mm mags for $77 shipped.
I look at it like fishing. You may not get a fish every day but if you keep looking you will get fish eventually.