The bolt size of our hardware is M6X40mm with a 1mm thread pitch. The stock bolts are also an M6 with a 1mm thread pitch; any length should work as long as it is between 20mm and 24.5mm long. Make sure whatever you use is zinc plate so it does not corrode or rust. Any hardware store such as The Home Depot will carry these.
Let me know if you have any other questions.