Unfortunately, Remington went into bankruptcy and stopped making them. Somebody bought Remington and have started production again, mainly starting with Remington 870s and other low fruit. Hopefully that get into full swing again and you can eventually get magazines. In the mean time, you will have to hunt for them on the 2nd hand market. Do some google searches. I got lucky years ago when Beretta stopped the original style 87 Cheetah .22lr pistols. One of the various magazine sellers put them on closeout to get rid of the existing stock and I was able to get them for $10-15 each, vs. the $35-40 Beretta was asking from them.
I got stuck with a Remington Tac-12 DM that I also got stuck without mags. So, I was in the same boat as you. I got rid of my Remington Tac-12 DM as I had little faith in the aftermarket making higher capacity mags for it. I put the money into an old Remington 870 Special Field that I will eventually convert to a SBS, which is what I was going to do with the Tac-12.