Sig Sauer Gen 1 MPX Magazine, 9mm, 30rd. These are most reliable in the first generation MPX. It is recommended to use the Gen 2 magazines in firearms with keypad rail (those are Gen 2).
Stock up on extra magazines for your MPX. This is a brand new OEM SIg Sauer MPX 9mm 30-Round Magazine. These are direct from Sig and not a third party or some other manufacturer.
Sig does also offer smaller capacity magazines but they may not be as readily available.