Where can I get Heki skylight hinges?

You should be able to find a stainless steel clevis pin in a size close enough. There is no reason to have a bolt, you just need a keeper ring to hold it in place. Maybe something like this? You would need to measure the size an pick one.

The hinges are plastic too - when my dome cracked rather that buy new hinges I was going to machine some aluminum ones, but ended up ordering a whole new skylight instead.