how to install outlet in the box that lacks neutral? my box has two black wires (hot and return/load) and bare copper for ground. it was designed for switch that controls light or fan (I guess builder didn't think neutral would be necessary in future). is my only option to run neutral from somewhere else? or can I use existing wires?
No, you cannot legally install an outlet without a neutral. Using the ground wire in its place is illegal and potentially dangerous. Moreover, the neutral wire must be in the same bundle as the hot wire to avoid eddy currents that could cause heating.
If the hot is in a conduit, you can simply pull a new wire. If it’s using NMB (Romex), probably -2 from the fixture, you’ll need to replace it with -3.
You need a neutral, so run a second cable to give the current capacity needed with a separate breaker if necessary, or add a neutral wire only if that meets code.