Buying a Morgan is always going to be a bit of an adventure compared to a "normal" car as there were never that many built: about 3513 4/4 1600 to go at. About half went abroad, so somewhere around 1500 cars somewhere in the UK.
Mine came from the classified pages of Motor magazine, (now defunct), but I know of people who have bought off eBay, their local free paper, craigslist and other slightly odd places. If you can cope with a car that needs work, makes sure all of the dealers know what you are looking for and let it be known at your local MSCC centre. After that, you need luck and patience.
