fastfr8r
I've grown lazy with my shorthand references
So have I, and most who write here. Not to worry.
Ithaca model 87, which was the 37 for a while
OK, got it.
I see a lot of vintage M-37 out there in the market place. Mostly hunting models, with standard magazine tubes. Most range in prices $200 to $350, check your local pawn shops and consignment / hunting oriented gun shops, I am sure one will pop up.
During my time in the Brown Water Navy, the M-37 was the backbone of our fighting scatterguns, a few M-12 and 870's mixed in. The M-37 is a solid platform and I like it. The shell lifter bars are of great use if you have a double feed. The weak link is the single action bar.
The problem I see, is that the present world of fighting scatterguns is dominated by two flavors of pump scatterguns. Remington 870 series(vanilla) and Mossberg 500 series (chocolate) every thing else is an afterthought for parts makers. All the tacti-cool stuff you may want to hang on your M-37 will have to be adapted or scrounged through very hard effort. The fixed mag tube and barrel locking stud will have to be dealt with, unless you are luck enough to find a extended magazine model.
Good Luck on your project, and Be Safe