Around home, you can look for any service station where farmers and/or loggers fuel up their trucks (with on-road fuel). For convenience sake they also buy their off-road fuel at these places. Just go in and ask them about off-road diesel. They may ask if you have a tax id (many of the folks buying off-road diesel from them are buying it for business purposes i.e. farming/logging), just tell 'em you'll have to pay the taxes on it.
BR