I have a Magic Chef forced air furnace/ac (gas heat, elec ac)
Model #G8A75DC42-17 and the fan stays on even after the ac turns off. I had an ac man out and they said the relay was bad but the unit is older and they could not get replacement parts for it. When I look at it, there is a little black box when tapped on, turns off the fan. Otherwise, I have to turn off the power to the unit to get it to go off. Is that box the relay and does anyone know where I might find that part? I can't afford a whole new unit and I don't want to burn up the fan either. Any help would be appreciated.

Replacing the relay is not a do-it-yourself project for the average homeowner. It likely will require a good knowledge of soldering - not to mention finding the exact replacement relay. Most AC techs are not interested in repairing a control board - they prefer simply replacing the entire board with as little fuss and muss as possible. Call around to various repair services, explain your situation and ask if they will repair the board. Sometimes, cleaning the contact points will buy some time - but again, not an average homeowner project.