Custom-custom? Or aftermarket?
Aftermarket can be had from CableMod etc, and will specify the exact PSU(s) that they're compatible with.
They're replacement cables for the stock ones.
Example: https://cablemod.com/product/cablemod-c-series-rmi-rmx-basic-cable-kit-white/
There are extensions too, that are universal and apply to any/all PSUs and are cheaper, but they're not quite the same level of quality (and harder to route as you're adding length).
Example: https://www.newegg.com/p/N82E16812229006?Description=cable extensions&cm_re=cable_extensions--12-229-006--Product
I got mine from this site - and they are very good as far as what you get goes:
Custom Sleeved PSU Cables made in the USA
Shop now for Custom Sleeved PC Power Supply Cables! Select from a wide range of colors produced by MDPC-X of Bavaria, Germany.mod-one.com
It's easier to buy a kit if you have a very common PSU like an EVGA G2/G3, Seasonic Focus / Prime, or Corsair RMx / AXi / HXi, but there are sites like Mod One and Cable Mod are out there that can let you customize your PSU cables down to the individual sleeve.