There's a good reason new Ithaca M37s are expensive. They are exceptionally well made. There is no aluminum or plastic on them. They are all forged steel. Even the trigger guard assembly is made of forged steel, not plastic like on new Remingtons and Mossbergs, and not aluminum like on BPSs. Additionally, their barrels are forged. The vent rib posts, the wedge on top of the barrel in the front of the vent rib, the lug underneath the barrel where the magazine tube tightens, the sling stud lug (on Deerslayer barrels), are all part of the barrel. The barrel and all those protuberances are all forged and machined out of one piece of steel. They aren't soldered on. New Ithaca barrels, IMO, are the best shotgun barrels on the market today. They are meticulously made.
I like the BPS, and I currently have one and have owned four. But in terms of build quality, the Ithaca is superior. When you cycle a brand new Ithaca and a brand new BPS side by side, you will immediately notice the Ithaca cycles like a gun you've owned and used for 50 years. In comparison, the BPS is tight and a little gritty and feels like it needs to be broken in.
There are plenty of good, reliable pump shotguns on the market. But the new Ithacas are, IMO, well above them all. I have owned several Ithacas, and currently have three. One a 1975-made gun and the others new production Ithacas. The new guns are the best made of them all, IMO.
There may be some retailers out there that stock new Ithacas, but I don't know who they are. AFAIK, you have to order directly from Ithaca and they ship to your local FFL. And new Ithacas aren't always available. I understand they are made seasonally, and they go fast.