Might be little tough... and you don't mention what scale....
But in either case, pickings are slim. H0 scale (1:87) might yeld some stuff as I recall seeing an ad for a New York City Subway cars (either plastic or resin kits) in the Model Railroader Magazine earlier this year.
You might be lucky in larger scales too, like O (1:48).
You best bet is to post your question on rec.models.railroad group and see if you get help there.
Peteski