Some listings on eBay suggest that the Pro Series M is compatible with Mossberg 500/590 pistol grips, but it's best to confirm with Black Aces or the grip manufacturer.
Specific Grip Options:
Hogue Tamer Pistol Grip: Known for comfort and recoil reduction.
Shockwave Raptor Grip: Popular for maneuverability.
OPSol Mini-Clip 2.0 Flex Adapter: Allows for use of standard shells in a shorter magazine tube.
Black Aces Tactical S4: According to their website, Black Aces offers this grip in various colors specifically for their Pro Series M shotguns.

Try here: https://cdarmssupply.com/black-aces-tactical

Hi Arlonta and welcome back. I would use the adjustable Pistol Grip Stock here: -- https://blackacestactical.com/product/s4-m4-tactical-adjustable-telescopic-stock/ ----------- Cheers and stay safe!

Hi Arlonta, you cab find Mossberg bird's head grips that fit the BAT Pro Series shotguns here >>> https://cdarmssupply.com/black-aces-tactical
Stay safe & have a great day!