Do you mean the bolt that compresses the spring? If so, then remove the bolt and measure the pieces. My best estimation would be 3/8" x 5" coarse thread. If someone knows better, please chime in!
A quick query of Lowes' web page reveals that fasteners in that general size and shape routinely sell for less than $2.00. I'm sure Home Depot, Ace, Tractor Supply, and others are comparable. I don't believe there anything particularly unique about that bolt.