Railroad Passenger Cars?

Hi: I have recently purchased and built (with lots of assistance from my Grandson) the Lego Cargo Train set...Really nice set and fun to use...Got a question: I would like to add a couple of passenger cars to my train set without buying another entire set...Are there plans available, with parts lists, for passenger cars that would work with my set? I do not have much of a collection of spare blocks and other parts, so I would have to order them individually..Any help and advice would be welcome..Thank you very much...

Steve and Kevin