Reenskaug hardware (bolt/nut)  

Post a photo of existing parts
and indicate your location.
It's likely that replacement parts, or serviceable substitutes, can be obtained off-the-shelf from specialty hardware suppliers like McMaster Carr.
Also, your nomenclature might be inaccurate. In the US, what you're calling a "bolt" is, I think, either a "low-profile socket cap screw" or "low-profile shoulder screw". I can't tell from photos what hardware is inside the seat frame or stretcher that the screws thread into, though. Could be "threaded inserts".