For shore power, and unless the Marinco system is something very special in which case I’d be inclined to change it, it’s a cable like one of these that you need. Don’t look for anything capable of handling more than 16 amps because most marina systems will not provide more current These are readily available on eBay, or from your local chandlery/caravan shop.
The price of Marinco stuff on eBay is beyond all reason !!