I have a friend who converted his diesel Volkswagen to greasel several years ago. He drove all over the country in it for free. I don't know if it's still the case, but a lot of restaurants used to have to pay to have their used grease disposed of by specialized companies, so they were perfectly happy to give it away for free for him to use as fuel because doing so saved them money. He said he'd just pull into restaurants where he saw the external used grease storage tanks (particularly common at fast-food places) and ask if he could take some. They said "sure" about 80 percent of the time.