Some old school BMX seat clamps fit. I have a couple of SR clamps on two of my Schwinn cruisers.
Not sure what other brands will work...maybe Suntour also? I believe the correct size would be the 1" inside diameter( as opposed to the 1 1/8" )
They take a one inch, AKA 25.4mm. I ran a clunky one from Porkchop on my old straightbar. Sooooooo many companies make 1" clamps, but mostly these are BMX companies.