How to compensate voltage drop on a wire
Get a power supply that has a remote sense feature &mdash; extra connections that allow you to run separate sensing wires to the point of load. Since these wires do not carry any significant current, they experience no voltage drop and provide accurate feedback to the regulator. When voltage drop occurs in the power wires, the regulator can sense it and provide extra voltage at the source to precisely compensate for it.

You can use a single supply if the motors are close to each other. If they are widely separated, you'll need a separate regulator for each one.

Edit: Obviously, there is a limit to how much drop can be compensated for; you also need to select wire that is appropriate for the current being carried.