Got a pic of the boards?
Most boards use the existing captured nuts which require a 1/2" hex head with a 1 - 1/4" overall length and either a 20 or 24 thread pitch.
Then, along with the 1/2" hex head bolts, some use a 3/8" hex head bolt about 1/2" in length to bolt to holes in the lower pinch weld. On this style, there are also captured nuts in the back of the mounting post on the step bar.
Lastly some use the captured nuts plus two new 3/8" diameter holes that you drill into the lower body above the captured nuts. In those holes you use a #14 x 1" self tapping screw.
So which bolts you need depends on which boards you have.