3M. They're not cheap. 300mm doesn't appear to be a standard length, though it seems they might make custom cables. Availability could be an issue--DigiKey being sold out of 5.0 cables until September.
I still have an ABIT IP35 Pro floating around somewhere. Ignoring the fact that ABIT went under in the middle of the product lifecycle (thus making CPU upgrades difficult if not impossible), it worked well enough.