MDPC-X - custom power cables, wiring, sleeving, DIY supplies. Not the cheapest, but one of the best and sexiest.
CableMod - more a prefab custom wiring company, has premade kits for specific and popular PSU brands
Generic bulk parts (screws, standoffs, etc.) can also be found for cheap on Amazon. USB ports (internal header to case port) are rather universal, so you can buy cheap case part replacements or expansion bay things and then remove the extra stuff you don't need. I've done this to upgrade the front ports on older cases, as they all had the same spacing, screws and housing.