Joe,
Welcome to the 48th scale world. There are a wide assortment of vehicles available. The problem is the most of the affordable ones are tanks (not much use in AC dios) or resin and metal ($$$). I just did a quick search at www.greatmodels.com and found these manufacturers.
Des Kits, Fuman, FM Models, Frog Models (old Bandai kits, have a jeep and kubelwagen that are nice), Fonderie Miniatures, FlightPath, Gaso.Line, Pend Oreille Models, Sol, and Tarmac. Tamiya has just released a couple of vehicles with their planes including a Kettengrad and US Navy tractor. Also just released a Kubelwagen that is very nice. Hasagawa and Fujimi have figure sets out for US Navy and Air Force which include a couple of vehicles.
'O' scale is actually 1/43rd scale so the figures and cars are a little large but look OK if not too close to the planes or any other 48th figures.