Cheaper here and add them in to your reloading/ ammo order.According to RIA it uses double stack 9mm mags, and they are available here:
https://armscor.com/parts-accessories/magazines/
ETA: I’ve ordered double stack 40/10 mm mags from them with no problems.
https://www.ammosupplywarehouse.com...page=product_info&cPath=292&products_id=20078
Mec-Gar doesn't instill a lot of confidence but the one I received with the pistol works fine.