You can find one on Amazon here
Camco 39452 RV Sewer Cap
https://www.amazon.com/dp/B000G72VHM..._VxVdBbWG89DB0
or here
Valterra F02-3106BK Black Bulk EZ Coupler Valve Cap with Handle
https://www.amazon.com/dp/B0006MRR2Q..._tzVdBbRNTMP7J
Or you can pick one up in the RV aisle at Walmart.