wolverine sells spear 5/30 mags that are drop in. Magpul G36 mags work with a slight modification to the mag. The factory G36 mags will work as well but require a small modification to the rifle because there is not enough material like in the magpul mags.
Switching to Stanag type mags on this rifle would be a mistake IMO since the G36 uses arguably the most reliable mag design out there.