If I recall correctly, the Ithaca M37 was available as a Stake-Out Special. Because of the way the barrel mates with the end of the mag tube, longer magazines aren't really practical so the SOS was a pistol grip version with the barrel cut off even with the end of the mag tube. I think it was Parkerized too. It was the smallest practical (5 shot?) tactical gun you could build but I think it was law enforcement only because the overall length was under 26 1/2".
Sorry, no idea where you can get pistol grips etc. My advice if you want to go tactical is sell the M37 and get an M870 and go from there or simply enjoy the M87 UL for the great gun it is and get another gun if you want something tactical.