Jon, I would try Numrich Gun Parts to see if available : gunpartscorp.com/gun-manufacturer/mossberg/rifles-mossberg/810

Found this place has the floor plate and spring youd still have to fi d the follower though
havlinsales.com/

I have an old western field 30-30 that is bolt action and magazine fed i lost one of the magazines and the other one ended up getting rusty so it would no longer reliably feed i was able to find a savage 340 30-30 magazine on eBay and it works i also found them on Brownells for cheaper after I already bought the one off of ebay.
I did find a site that sells about every part to a mosberg 810 but thye don't sell whole magazines they sell the shell, spring, follower, floor plate, and floor plate latch separately, current they are put of the spring, follower, and floor plates but they do have th latch and shell in Stock, id get on their notify me when available list and you'll be alerted when they get the missing pieces in.
gunpartscorp.com/gun-manufacturer/mossberg/rifles-mossberg/810