There are a couple different types of trucks, but in the interest of saving you time and money I recommend you purchase a used Chevy or Grumman-Olson step van. The majority of food trucks are step van postal trucks formerly used by DHL, FedEx, and other large commercial companies. These trucks are large enough for an average-size adult to stand upright and to house all the necessary equipment. Look for a non-diesel model that is approximately 14 feet long and 9 feet wide with less than 100,000 miles. The main sources for purchasing a step van are eBay, Craigslist, truck dealers, truck auctions, and customizers. Make sure you check them all out to get the best bang for your buck.
Once you’ve purchased your truck you need to make it a food truck. The first step in doing this will be finding a customizer in your area. In most major cities you’ll find professional food truck customizers, but all you really need is a quality sheet metal shop. I recommend finding someone who has experience working on trucks and is aware of local health codes.