S-Lon, size #18 in a single pack of black or white.
For Kumihimo braiding, S-Lon (Super-Lon) in size #18 is the "regular" size and works perfectly when using size 8 or 6 seed beads. Multi-packs are cheaper than individual spools, and Amazon has very competitive prices. Every spool has 77 yards of thread, no matter the size of the pack. I have put together this selection of 1-spool, 4-spool and 12-spool packs, all with free shipping.