It's a metric bolt and I'm not sure of the size, but I'm sure you can find one at your local hardware store.... Get yourself a new grade 8 replacement and that will hold up. Your typical seat post come with a 10.9 grade bolt and even a 12.9 does not compare to a grade 8.

http://www.boltdepot.com/fastener-in...ade-Chart.aspx