RV outdoor power cord will do the job.
Here's 4 wire, 50 amp. You'd need to change to marine ends. Amazon:
https://www.amazon.com/NU-CORD-9456...8&qid=1501436193&sr=8-15&keywords=50+amp+cord
Also try Ebay.
Paying for an RV cord and paying for two marine ends is going to cost more than buying the proper cord in the first place.