Oklahomaguy is right on the money. You can obtain an M2 magazine catch from either Fulton Armory or Numrich Gunparts (always an excellent parts source) will serve you well. (Numrich has the part in stock as I write this).
www.keepshooting.com has some 30-round magazines available. Not only do they have decent reliability ratings, the magazines have a "last round hold open" feature. At less than $16.00 each and a flat $9.00 shipping rate, you should be quite satisfied).
I have some that will arrive in my mail box on Wednesday morning (8/10/2016).