Here's a few suggestions:
870 Police Mag. USSS - Auction: 8593060 (Ends 04/28/2008, 1936 PST)
W/GHOST RING NIGHT SIGHT, SHELL HOLDER GORGEOUS... - Auction: 8576640 (Ends 04/27/2008, 16:44:29 PST)
870 Police Mag. AOW - Auction: 8593061 (Ends 04/28/2008, 1936 PST)
Or buddy up to someone with an FFL and have them give Remington a call for you.
From a reliability point of view, I seriously doubt the 870 has much of an edge on the Mossberg 590, especially if it's not going to see constant use.