'03 without cruise, check again.
Jack what's unique about your van? Is it a fiat?
Every vehicle on the road has a cruise option offered in their option list, either for domestic or export markets.
IF you have a good dealer who has a good parts man that you can depend on, have them run through their parts catalog to find the components for the cruise control and get it printed off with the diagram. It can be done regardless what they tell you. IF they don't want to do that, call up Ford/GM customer service and ask them for a phone number or an email address to send a request for a parts list for the option. I have to tell you that the person who you talk to either in the parts department or in customer service will tell you that you need a VIN number but that's bull - they can search by option number.
Once you do this, then get onto one of the used parts sites and start looking for vans, passenger vans will most likely have cruise as part of the option package.
Fly by wire is the easiest to install, even the gas engines. It is controlled through the ecm and not the throttle cable - so it is a couple switches and other components and the dealer turning on the option.