I picked up a couple of 5 round stainless steel mags from here. https://store-5li2gj.mybigcommerce.com They have worked flawlessly with 5.56 and 300 blackout.
Lapolicegear.com just had an e-mail with C-products brand 5 rounders for both 5.56 & .308 on sale. These were the metal type & not polymer though. I have 2 each, 5 & 10 rounders. No problems.