The MP35i magazine is proprietary- It is not interchangeable with any other magazine without modification. The MP35i mag is a double stack, double feed magazine. The MP28 uses a double stack single feed, like the MP40. The closest mag is an MP34 Steyr, but you need to solder a backing plate onto the rear of the mag to act as a spacer/stop to keep it from being over inserted and obstructing the bolt travel. Some folks have modified Uzi magazine to work with the MP35i, but they look bad and obviously not original.
I bought a MP35i kit without a mag a few years ago, and I found that MP35i mags are rarer then almost any other WWII German SMG magazine. At least 20x rarer then MP38/40 mags, when I compare the numbers of MP40 mags I have seen for sale to the MP35i mags that have surfaced for sale.
I still think $900 is too much.... And that mag has not sold yet so the market is speaking for itself.