I agree with Ed, White Ensign Models is your best bet if your want crisply cast resin vehicles, but Loose Cannon Productions http://home.earthlink.net/~loosecannonproductions/ makes some white metal ones that are decent.
There is also a Skywave detail set, SW06 US Beachead Vehicles, in 1/700 that has Sherman tanks, 2 1/2-ton trucks, Dodge command cars, LVTs, LCVPs and barrage balloons. That may be the cheapest option if all you want is a few tanks and trucks. It even has itty bitty white star decals (which was why I bought it).