If you've got a tap and die set use the thread measuring tool to see the thread type. Going to a proper hardware store (a real one) would also be an easy solution.

nlangley, I note that the above is your first post on this forum. Sorry you have encountered a minor problem with fitting your seat, but really? How about taking the bolts to a decent fastener shop and asking them? If you are working on your bike and encounter this sort of problem and then feel the need to post up on a forum, then I have to question whether you should be allowed to use a knife and fork, let alone hand tools. If it was me, I would go to the large container of assorted nuts and bolts I have in my garage and find some that would work.......

Don't take it personally mate, you now however have the mental tool to think before you post. It's all good though mate and welcome to the forum.