Are you looking for a newer van (as in, say, 2008 or newer) or an older van, say 1992-2007? They were the same basic body style from 1992 up, but I would recommend looking for at least a 1997 up with the updated dash, OBDII (for a scanner or tuner, which started in 1996), etc. I would post what exactly you are looking for and someone here will help you find it. Sportsmobile Forum or Ford Truck Enthusiast forums are also good places to look and ask. The 'found vans' thread here (in the 2/4wd camper van section) is probably the most up-to-the-minute place to find vans, and you can take note of a few users that are helpful and always finding stuff, Bbasso comes to mind. When searching Craigslist I search 'Ford E350', which is the full one-ton van that a majority of conversions or what you would want to convert will be. You can't do much with an E150 because of the weight limits and that will mostly show you beat up white work vans or ugly conversion vans. E350 is where you will find stronger power plants, such as diesels and V10s, and better contenders for conversion whether you want a passenger (club wagon) with all the windows and rear air or a cargo van.
Good luck and don't be afraid to ask questions. PM em if you feel silly. We're all van geeks here.