You can win the truck races with any of the trucks in the game if you upgrade them enough. So as long as you can get a truck, ANY truck, you should be set. If you don't have online, I'd say go bronze all of the I-A license tests. If you can't do that, your only option is to keep cycling the used car dealership until a truck pops up. But getting all bronzes on those license tests is a pretty easy thing to do, if you can't do that then I think you may have a bigger problem than not being able to find a truck lol.