I don't know if they still have it because I haven't bought it in a while, but I used to get my magnet paper at Walmart. It's made by Avery and comes 5 sheets to a package. It was with office supplies. I've also seen it at Staples and Office Max. I've never seen the ProMag stuff. If it's made for printers, I would think it would be fine though. Just make sure you only put one sheet in the printer at a time.
I've seen the Avery at Office Depot the other day for $15.99 for the 5 sheets. I don't think Walmart carries it anymore. I saw the Promag at Michaels. The only problem I would see is that the paper is rolled and don't know how well it would straighten out in the printer. I found a website that sells the magnet paper decalpaper.com. I looked around and they were the cheapest. I ordered 50 sheets for $55 ($1.10/sheet) considerably better than the $3 at Office Depot. They are not the Avery brand though. I'm expecting shipment next week. I know you can order 5,25,50,100,etc. I know the 5 was $9.99. The more you order the more the price goes down. I have to print alot of signs for 2 cruises so that's why I opted for the 50. I'll let you know how it prints. Hope that helps.