I need toddler tractor underwear! Where can I find them?

I have a two year old that is obsessed with tractors. He is starting to potty train, but won't ditch the diapers for anything! He screams at the thought of putting on undies or pull ups, but wants to pee in the potty all the time. He loves tractors so much that I know he would wear them, but I can't seem to find them anywhere! There have to be toddler tractor underwear out there somewhere!!