Wait till the 50's, find converables, sell them for about 2k. Also, if you find a car that is called a "Hollywood" or something, you can sell them for over 2k. The Hollywood is basicallly an old version of a limo. Leo gets driven in one.
Also, the car used in the mission where you kill "an old friend" can net you just over 1k a piece.
To find them, you can drive around the docks area or the richer area of the inner city near either the Falcon restuarant that Eddie opperates out of or the rare, rich clothing store.