Its usually called "garage keepers" coverage. Your own auto insurance won't cover it because you're driving someone else's vehicle. Its considered a business insurance, but you can get by with just the garage keepers. Contact your auto insurance company and ask about it. I made sure I was covered for the entire amount of the most expensive vehicle you plan to detail or transport, just in case.
My coverage was about 1300/yr, so just over 100 bucks a month, which is just 1 detailed car. So basically for a detail a month, you have the peace of mind knowing that you're insured.
