The Bioloid kits ship with a vast number (200?) of 'S1' bolts (machine screws). S1 are used to join brackets to servos, and some brackets to brackets. S2, S3 and S4 are really useful when two brackets need joining. But Robotis only include 20 of each! This isn't enough for 4 per leg on a hexapod. You can buy a complete set of additional bolts, but you still only get 20xS2, 20xS3 and so on.
Fortunately the bolts are standard sizes:
S1 = M2 x 6mm
S2 = M2 x 8mm
S3 = M2 x 10mm
S4 = M2 x 12mm
So a quick eBay search for "M2 X 8 PAN HEAD POZI" turns up packs of S2. The ones I bought from jf_tools have a slightly larger head than the Robotis ones, but in many places this is a benefit (they sink into the plastic less when done up tight)