I've been trying to find a backup magazine for my12 gauge Marlin 512 slugmaster. EBay mag pricing was going for $25+ to $40+. Used mags were going for similar pricing as new. Crazy.
Found out that Brownell's has them, new, and for only $22.66+ shipping. Here's the link for the 12 gauge mag. Not sure if they have them for other gauges.
Just received my shipment last week. Well made, with a strong follower spring. The description says its made by "Marlin." It looks as good as my original, so I believe it. Buy one now, who knows if they will have them in 5 years. Numrich Arms/Gun Parts were out of them.
Thought I'd share the source with you.
Free advice is worth what you've paid for it.