Underwear

Any idea on where I can get small underwear? We’re getting ready to pull the plug on diapers and officially start potty training my son soon. I wanted to let him pick out some big boy underwear, but the smallest size we could find was 2t/3t with a weight minimum of 27 lbs. He’s a peanut and still wears 9 month shorts and 12 month pants because of little waist. He hardly weighs 22 lbs! It seems like all the fun underwear he wants do not come in small sizes...