[DonRotolo]

It is possible to make such a power supply, but you'll need someplace a bit more well-stocked than a Radio Shack

A 36 Volt 3 Amp (or better, 4 Amp) transformer converts the 120 AC to 36 V AC.
Then you use four diodes to make a full-wave rectifier, converts it to about 30 volts pulsating DC

A large capacitor (20 uF or so at 50 volts) removes the pulsations.

You then use an adjustable voltage regulator rated for 3 Amps (such as an LM150, but there are dozens of acceptable devices) to bring the voltage to 24 volts. Some small capacitor at the output (depends on the regulator, but maybe 1 uF at 36 volts) and you have a 24 volt 3 Amp power supply.

I could build one from my junk box, but I'm a bit far away. Maybe a nearby team knows of an electronics store - even a TV repair shop! - that can probably help. My best suggestion is to not panic, but get moving.

If you need help, PM me, but your Engineer friend can probably design a simple power supply around whatever's available in a minute.

In a pinch - I think this qualifies - you can connect two 12 volt batteries in series to get 24 volts of very clean power. FIRST batteries may last only an hour, but two car batteries should last a few hours at least, maybe a whole day.

Please let us know how you make out with this.

Don