Solution for those odd ball sizes... Take a small diameter pipe that fits into the frame easily and wrap tape around the end till slides in and out of the frame snugly but not too tight - Measure the diameter (24mm)

Buy a seatpost of your choice slightly larger in diameter (alloy 25mm about 18$)

Take the pipe with the tape and the slightly oversized seat post to a machine shop and have them turn it down in diameter (10$) - Loss of 1/2mm in wall size is not significant to affect its strength...