I've got an old Ithaca Model 37 and like it quite a bit. Parts and accessories are nearly non-existant, but I really like the action design. With the exception of the previously mentioned rattling forarm, it feels very solid. It's weight is about half that of a Mossberg 590A1.
I was looking for one a while back, also. I was able to locate a few "riot" Model 37's with, side folding bayonet, 7-round mag tubes, and 18 inch barrels, but the dealer wanted a bit more than I wanted to pay for a LE/military retiree ($500).
I know the Marine Corp and the SEAL (duckbilled model '37, AKA "Sweet Lips") teams used the model 37 during Viet Nam, so you figure it had to be pretty decent in that role.
If you want a cheap Ithaca knock-off that appears pretty well built, check out the Brolin Arms YL-12. It is Chinese made, has a 18-inch barrel, 7 round mag tube, and cost is about $130 on sale. Sadly, the barrels are not interchangeable, the Chinese version uses an unthreaded set of lugs to attach, whereas Ithaca used interrupted threads.