Anybody know a source for Lionel's Baby Madison passenger car interiors?

Have you tried Lionel?

try here;

http://www.traindoctor.com/

The most recent versions of the Baby Madisons have interiors and clear windows. You might check on line at the Lionel site for parts for recent Madisons, and see if they're available.  Good idea to check with the Train Doctor at Brasseur's, too, if that doesn't work.

I wonder if it wouldn't be cheaper to buy the new cars and just sell the old ones?  I'm guessing that buying significant parts like that will get expensive.

Taking a quick look at the Lionel parts site, it doesn't look like interiors are available for the Baby Madisons. Actually, it seems like many of the parts for these cars are unavailable, although the clear window strip parts are showing. Looks like the only option may in line with gunrunnerjohn's comment - have to buy new cars to get the parts.