Where to buy monopoly sized 1920ies plastic car

Hmm, I wasn't able to find exactly what you wanted, but here are some things that are close.

Wooden cars and probably not really what you are looking for, but it's a start, click on Standardprogramm->Figures->Fahrzeugen : http://www.lorenz-holzspielwaren.de/

Another nice one I found at http://www.dice.co.uk/fs_gamcom.htm . These are actually plastic, but I don't know if they are gangster-y enough.

For more game bit companies look under "Web Resources".