Over on Gunbroker, here's a list of all 20 gauge 828u's for sale:
https://www.gunbroker.com/Shotguns/search?Keywords=benelli 828u&PageSize=96&Sort=1&View=1&Ch-Gauge=20 Gauge
Here's a used one in Lake Benton MN if that is anywhere close:
https://www.gunbroker.com/item/1006809290