Where do you find commercial automotive liability coverage?

Liability coverage
This is a very odd request from the leasor. Unless you and your friends are incorporated or considered a business, no insurance carrier would sell you this type of policy. Also, any such coverage would probably duplicate the liability coverage you already carry on your vehicles.

Instead, you might ask the leasor to consider writing in a "hold harmless" agreement in your lease. This basically places any liability back on you, and keeps the leasor safe. Quite frankly, given the situation you described, I can't imagine what the leasor needs to be safe from, unless it's a case of off-roading and the land is open or available to the public. It's possible the leasor is worried that the land is an "attractive nuisance," meaning people can enter it freely and run the risk of being hit by a vehicle being driven by you or your friends.

Try the "hold harmless" agreement first, though. That usually does the trick.

More input from FAQ Farmers:
For bus and 15 passanger vans 5million dollars coverge