without any money for paint I would think you have no money for all substitute ways either. So find a graphics company that uses trucks for their needs and trade them out. Your services for theirs.
Look into vinyl decals. They will lst 5 or so years and are way less expensive than painting, or you can also look into body wraps. They are also vinyl that wraps the entire truck with your logo.
Consider getting a magnetic sign for the door of your truck s cabs. That would be cheaper than painting a whole truck. If you are looking for money, there isn t any except what s in your pocket, bank account, or credit card.
Perhaps you could hire a spray paint artist, someone who could help you come up with a cool design and then paint it for you cheaper than a professional service.