At one time the Ingram M6 mags were made of unobtainium. Hard to find and awful pricey. Even now they are not cheap, APEX lists them at $122 plus shipping.
As others have found out the Thompson magazine could be modified to fit.
Dad modified a number of cheap( $5 ea or less) Tommy gun mags to work in his M6. He made a couple of side by side mags for quick reloads.
For casual sub gun competition the side by side mags were great for a quick reload. To prevent the cartridge from walking out of the mag Dad fabricated a small piece to act as a stop. See pic.
We tested the gun and all the Thompson mags ran flawlessly. One of the original M6 mags did not, seems they are made of thinner steel than a Thompson mag.
The M6 is an interesting gun, too bad it was not introduced in 1939 instead of 1948 !