Checkmate and Thermold are the only ones I use with my M14 EBR.
https://checkmatemagazines.com/product/m14-m1a-magazine/
https://www.cheaperthandirt.com/thermold-m-14-and-m1a-rifle-magazine-7.62x51-.308-winchester-holds-20-rounds-black-zytel-nylon-magazine/FC-895389002222.html