Constant DC 12V, 2A from USB 3.0, SATA, Molex or other common PSU cable?
The power connectors in a PC supply are directly connected to the power supply. There is no negotiation required, unlike USB.

A "Molex" 4 pin power connector or a SATA power connector do carry 12V multiple amps regularly. How consistent the 12V rail is depends on the load and quality of the power supply, but you shouldn't have any issue with that.

If this is in an active computer, it will only work when the computer is on. Or if you pull a computer power supply out, you will need to add a power enable switch, and a sufficient load for the supply to maintain power regulation.