5/16 -18 is the thread size. Do be careful with our import buddies as they sometimes screw up (no pun intended). Any local home improvement store will have what you need.
FYI 5/16 is the outer thread diameter, and the 18 refers to 18 threads per inch.

if the original bolts thread in easily you likely have been bit by "you want cheep, maybe wuhk, or gualantee to wuhk"