HELP! looking for a mag to fit the Magpul receiver

I’ve recently had the same issue of fitting magpul mag (from my acr) in my gen9 m4, turned out it was one of the notch sticking out on the side that stops the mag from sitting in properly.

Turnes out my spare mag for the acr doesn’t have those notches (will upload pics in a sec), and they fit just fine

the very top notch that sticks out on the ridge of the mag.
pmag with out it should fit any of the ar platform mag well

Where can I find this mag?

I am no sure exactly where it came from as I got my acr second hand with an extra mag.
I am guessing ebay? Or most blaster store should sell mags.
Just make sure it doesn’t have that notch

Just file down the notch?