As far as I know ther are no other mags or clips that are compatible with the Marlin 50. When the Model 50 was changed to the A1, Marlin redesigned the mag. From what I can tell from research, there has been no aftermarket mags made for the model 50, Ever! That's probably why the last one that sold on Gun Broker went for $200.
You will just have to keep looking and try to luck into one but it is going to cost you!