Found three M37s on the first page of the search I did.
http://www.gunbroker.com/Auction/Vie...Item=102656657
http://www.gunbroker.com/Auction/Vie...Item=102150774
http://www.gunbroker.com/Auction/Vie...Item=102160252
As for 870s, they are harder to locate since there is only 7 years of C&Rs out there. YOu'd need to to look at the pictures/text for serial number or email the seller what serial number it was.
572,000 is the cut-off for 1957 870s.
Ithacas cut off at 704,000 for 1957 M37s.
As a price point, I paid under $200 shipped for my 37, and got a couple 870s for $280 & $320 shipped.