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Two options. Contact Bob's Gun Shop - their web site shows they have them:
Link to Bob's Winchester Shotgun - search for "37 Single"
You can also contact Macon Gun Stocks - they have forends they make. They can sell them to you to fit and finish or they can do that for you for a fee:
Link to Macon Model 37 forend
I hope this helps.
Please rate my answer on finding a forend for your Model 37 using the provided button. That will close out the question and also allow me to be paid. Thanks!