Iride01 

Thanks for the other pic, that clears things up quite a bit. Although you showed both sides in the first two pics, I didn't swap the bolt to the other side and realize that there is a head on both sides.

I've had seat post clamp bolts like that. They were the ones I typically broke often as the threaded part is so much less diameter than the other part it threads into. Though I don't remember any bikes having one that would span that distance. However a couple other members gave you good hints on what to look for and search for on the internet.

You also might just go down to your local dealer for that bike and see if they have or can get some for you. But if you have to improvise something, stainless steel would be my preferred material. 316L if you can find it. Maybe at a marine supplier if you just decide to go with a regular bolt and nut. 304 stainless, which is the common SS you find, will in areas of salt breeze not hold up as well.

Still, if you take that out. Clean it up with a wire brush and spray with some zinc based primer will likely last a decent amount of time with a touch up every great once in a while. Just don't over torque it and break it when you put it back in.