Glad to have you here on iRV2. I do not know of any specific locations for dinghies, other than the few that may show up at dealers.
Why not just find one you like and add the tow base, tow bar and braking system. You can find used towing equipment at Hitch Trader.
With an exosting dinghy you will not know how the previous owner treated it when towing or even how far it has been towed.
When we had a motorhome and a dingy, the car had 50,000 miles on the odometer from driving, and had been towed another 60,000 miles. The suspension was showing the excess mileage and it was not as tight as a car with only 50,000 miles.
We lived in Thornhill, Ontarion for 2 long and cold years...I sure do not miss the cold and snow.