Good auto parts stores should have them too.
And the butterfly valves are always messy - if you have to roll it outside, it never gets drained. Pull that stupid valve, put on a street elbow, a short nipple to get clear of the tank, and a regular
1/8" or 1/4" ball valve and a barb fitting where you can attach a hose. Then direct the hose into a bucket where you can catch the muck water, or on a semi-permanent under the workbench route the hose outside.No, you didn't actuate the unloader by having the pressure switch hit the high 'Off' setting - someone set it too high. And when you tried starting it by just plugging it in again, the motor was still fighting against head pressure and stalled.
Now you know what Locked Rotor Amps means, and you might have to find and push a Reset Button on the motor.
Adjust the pressure switch so it kicks off when it's supposed to - about 105 to 110 PSI - and try it again. Follow the cord, you'll hit the pressure switch. Pop the top, there will be an adjustment screw or nut dead center, and usually a notation as to which way is Up and Down.
If the Unloader valve sits there and hisses and the tank slowly bleeds out.... You need a new check valve going into the tank.
Standard item, should be right next to the safety valves at Lowes. But call first, they can tell you. It's either 1/2" NPT in and out, or
5/8 Copper Tubing in and 1/2" NPT out.
When you get it back, if he gets the pressure switch set right and the unloader working, and it still has starting issues, see if the motor has a Start Cap or a Run Cap. Cheap to try, often they get dried out and go bad after 30 years.