Check out the robogear line of toys/models. You can find them on ebay pretty cheaply. Google will point you to a number of orky conversions using robogear vehicles.
As others have said, it's can be just a matter of taking toy trucks and orkifying them so they look less 20th century. The easiset way to do this is by adding armor plates.
This article here:
http://chicagoskirmish.blogspot.com/2011/12/28mm-on-cheap-toy-cars-as-wargames.html
was written mostly with post-
apoc in mind, but it shows how to do cheap armor plating from blister plastic and a 1/8 hole punch.
Lastly, the biggest give-away that a toy is not
40k is often the wheels. If they're too realistic it's a dead giveaway of a modern car.
I recommend either finding a truck like the ones pictured above that have really crude wheels and then shaving off the writing.
Alternatively the cheap car-board books that are often available for a buck are a good source of nice chunky non-realistic wheels that look plenty orky.