I have several of the Marlin Factory 20 round mags..I Found them on gunbroker.com. (Also try Gunauction and those used gun websites)
The knock off cheapo ones are crap. I had one, cost $20 , used it twice. It jammed everytime. Threw it away.
The S&W 6900/5900 mags are also good quality
I've had Camp 9 but only had one stock mag that came with them. I also had several 45acps and although I never had Hi cap mags they worked well with GI 1911 mags, Colt 1911 mags and some other hi end 1911 mags. I have had no luck with the $40 after markets for Brn HPs, Lugers, and mags for a assortment of obsolete 32 & 380 autos. Their mags for 22 pistols are complete trash. When I'm at shows and sales I buy any magazines that are $5-$10, even if I don't know what they are for. As long as they look original.
I never did find any when I searched 5-6 years back. I just stocked up on 15 rounders. I bought some 30 round mags online (forget where now) but for something like $20 bucks apiece, they worked fine. If I figure out where it was I'll let ya know.