Electronic – Why can’t you use the ground of a second DC power supply as a return

The only places i've seen wall wart supplies with an earth ground terminal are on supplies built for the alarm system industry and normally only on the 16.5/24V AC supplies.

Here is an example

As to why you don't see them. The ground terminal on mains power is intended to be used to earth ground the enclosures of devices operating at mains, or high voltage. For example a blender with a 120V motor and a metal case. This prevents the chassis from ever being at dangerous voltages due to wiring faults or shorts within the product.

DC supplies normally have a floating ground that can be at a much different potential level than earth ground. There normally isn't a use for a earth ground in such systems. You'd have to be very careful with it to not cause a ground loop and grounding the chassis to the floating ground is normally done but more for emissions issues as unsafe chassis voltages aren't a concern in low voltage DC systems.

You should be able to find cheap 48V DC power supplies on e.g. ebay. Those are commonly used in industrial applications. Not clear if you are looking for an adjustable power supply or not...

If you have a floating power supply you can also connect them in series (e.g. two 24VDC supplies in series can give you 48V DC). If you have two lower voltage adjustale floating power supplies (most lab ones I've seen are) you can use them in series to get adjustable higher voltage. Shouldn't be too expensive, a quick google shows you can get a 30VDC adjustable supply for less than $100.

Building your own adjustable DC power supply is going to be a bit more complicated. If you go that route by the time you're all done it will cost you more than buying one (though you will learn a lot). Cheapest option is probably a bunch of 9V batteries in series (just don't short them).