Does anyone know I can find a flatbed car that either has a 1940 ford on it or that I can put on it?
There are LOTS of flat cars. 1940 Fords are a little rare. Collecting 1:87 vehicle models has almost become a hobby unto itself, so there are lots of models out there, but the closest I could come to what you're looking for is a 1936 Ford Fordor by Classic Metal Works/Mini Metals. Check with your LHS. Good hunting.
Old Timer
Scale Structures (http://66.241.223.134/merchant2/merchant.mvc?Screen=CTGY&Store_Code=jaks&Category_Code=AF100) has 40's and 50's vintage vehicle kits. No 40 Ford however. Good quality cast metal kits though.
I think that you could use any bachmann flat car (unloaded of course)
http://www.hobbylinc.com/htm/bac/bac18349.htm
http://www.hobbylinc.com/htm/bac/bac17334.htm
just get something to hold it in place.
Jordan makes an HO 40 Ford (2 door) kit. Requires "fine tuning" of parts before assembly and painting.
Walthers, as well as other suppliers, have it.