https://shop.ammocanmagnets.com/
I'm on an ammo organizational kick, and was going to place a fairly large order for magnets from the link above. But, I wanted a few custom magnets that he didn't already offer. After 4 emails and a phone message with no response, I didn't have a lot of faith in his customer service. So, I bought the stuff to make my own.
I bought a pack of (10) 8.5x11 sheets on Amazon for around $18 shipped (search for Marietta Magnetics - printable magnetic sheets; won't let me post a link for some reason). That comes out to roughly .15 per 50cal magnet or .09 each for the 30cal magnets (compared to his price of $1.50/1.00) plus the cost of ink.
I've created templates for both 30 and 50 cal cans; you can put 20 magnets on a 30cal sheet and 12 magnets on a 50cal sheet.
These will print on either a standard inkjet or laserjet printer, but here's the catch: it has to be a printer with a straight-through paper feed...like the older, desktop printers. If the paper is stored in a "magazine" (like a pull-out tray at the bottom of the printer), the magnetic paper is too thick and will get jammed in the rollers of the machine as it feeds. I learned this the hard way after WalMart, OfficeMax, Kinko's, etc. declined to print these for me. In the end, I bought a cheap (<$40) inkjet printer from WalMart and it works just fine. I needed a printer at home anyway, so it was an easy decision for me.
If this is something you'd like to try, PM me an email address and I'll send you the templates; they're in MS Publisher.
-WRM
