What I did, prior to finding my mossberg magazines, was use a Remington 597 magazine. You can also use the Marlin 795 magazine. You'll need your donor and a factory 10 round magazine for the mossberg. You'll use a dremil or similar tool to carve out the feed from your off brand magazine, and then got off the bottom of your Mossgerg piece...then you just glue them together.
You'll use the slide from your mossberg with the spring from the high cap magazine. It doesn't work great, but it does work. Better to stalk your local Academy stores for 25 rounders....they still have a few.