You mean the bolt in the frame? that is normally called a seat binder bolt. There are several divverent types some have a 2 parts and use a keyway to keep the 'nut' portion stationary others like the Campi ones just have a small knurled area and use friction to hold it. Some are just screws/bolts that simply thread into the opposite 'ear'
Post a few closeups. You may need to check places like Harris Cyclery.com, EbikeStop.com, Loosescrews.com or maybe Bike tools ect.
__________________
“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk