Look on Ebay. There are a bunch on there.
Not all Ithaca model 37 barrels are interchangeable.
Depending on when it was made will determine what will fit.
The early guns need to have the barrel fitted to the receiver.
Most late model guns will interchange, but not all of them.
If the serial number is below 855,000 the barrel will have to be machined and fitted to the particular receiver.
Also once the new barrel is fitted to the receiver the original barrel more than likely will no longer work on that gun.