Where can you buy off road diesel?

You need to call around to fuel stations and see if any of them carry it. If you're in an agricultural area, it'll be more commonplace, and a lot of coops will carry it. Otherwise, you might have to rely on a bulk fuel supplier to provide it. If you're absolutely unable to find it, and are using it for a legitimate purpose (and not trying to evade fuel taxes by putting it in an on-road vehicle), you can always go to a truck stop or fuel center which caters to trucks, and explain to the attendant (BEFORE you take on fuel) that you're fueling a piece of equipment which is eligible for non-taxable fuel, and they should be able to write it up as reefer (refrigerated trailer) fuel (which is also non-taxable use), and you'll be refunded any road use tax paid on that fuel.