They're readily available, halfords, eurocarparts, amazon, ebay........ Just Google 'tow socket converter'.I have just bought an older caravan and need to get a towbar fitted. I know that you can only now get 7 or 13 pin sockets on the car. Not sure if the caravan is an old 3 pin plug. Can you get an adaptor to convert from the 13 pin to old style 3 pin plugs.
I’d go a stage further, whilst having your towbar fitted and 13 pin electrics as suggested get the chap to fit the caravan with a 13 pin plug. As Clive said not expensive and not difficult to fitFor the car I would go 13 pin every time as they are more reliable. Then use a converter for connecting car to caravan. That way if you buy a later van it may well have 13 pin fitted to it so you can dispense with convertors. But it’s not expensive to convert van to 13 pin too.