I agree that conversion of 1/43 scale cars is your best bet. I've used 1/32 occasionally in the past, but I'm moving back to 1/43 and 1/48 exclusively. Other than that, in 28mm scale, you're mostly limited to aftermarket ork vehicles.
Here's some of my converted toy cars and articles on construction. Bigger and better pics at each link.
Hot Wheels motorcyles for 28mm figs.
http://chicagoskirmish.blogspot.com/2012/06/post-apocalyptic-cars-and-motorcycles.html
A MadMax'ish "Interceptor"
http://chicagoskirmish.blogspot.com/2012/04/last-v8-interceptor.html
Monster Bug, Old PIckup and lightly dusted Hummer
http://chicagoskirmish.blogspot.com/2012/03/28mm-on-cheap-sci-fi-bikers-and-post.html
Tanker, Dump , Silverado and Rover
http://chicagoskirmish.blogspot.com/2012/01/28mm-on-cheap-post-apocalyptic-vehicles.html
SUV and Microbus Full conversion instructions.