My BFG had one, Asus has one, Sparkle, Gigabyte. XFX doesn't include one, nor does MSI I think. Don't know about eVGA, they didn't offer one with the 9800GX2.
You can buy either type. There's 2. One is 2x6P into 1x8P and the other is a 6P -> 8P. If you get the 2x6P -> 8P, you'll need another converter for 2x4P -> 6P!