I understand how you feel about the price for the magazine, but that may be what your looking at.
You might check here, if you haven't already done so:
Go to the Mossberg web site.
Click on "Resources" At the top of the page.
Click on "Collectors Association"
Click on "Mossberg Parts"
You will be linked to Havlin Sales & Service. They seem to be the experts on Mossbergs and sell parts.
Also, there is a rather active Mossberg forum over at RimfireCentral.com. There seems to be a few folks over there who are pretty resourceful when it comes to Mossbergs.
Good Luck!