You can check out thrift shops in your area. They will often have bags of old toy cars selling for a song. Bring along a sample or two of your own vehicles or miniatures so you can check scale. At the price, you won't feel too bad taking a hammer to them to make them into wrecks! I've found suitable toys at Value Village, Salvation Army and other charity or thrift shops.
As has been mentioned before, the Pixar Cars toys are approximately 1:55 scale, and the line includes some old-timey vehicles - although they may be more 1950s than 1930s. There are other, OOP diecasts if you can find them, such as Matchbox 'Models of Yesteryear' which tend to be a good fit for 28mm miniatures.