Midway is a good source. www.midwayusa.com
Have you shot a pistol grip shotgun?
They are short... good.
Kick visiously, and are virtually un-aimable... BAD
Do you have to AIM a shotgun???
00-Buck from an 18.5" cylinder choke barrel spreads at about 1" per yard. 10 ft... 3" group... About the average grouping for a pistol under stress. 20 feet yields a 6-7" pattern, so yes, you can MISS with a shotgun.
Before spending big bucks on a pistol grip, shoot one first, and attempt a real-life scenario defensive shot inside 10 yards, with a follow-up...
You'll be glad you have a stock.
If you choose to go with the pistol grip, there are plenty of guys on here that have bought pistol-grip versions, who will happily BUY your 870 stock.