networking - -48VDC power required: How to make it work with 110VAC input

I recently bought a used NetOptics TP-GCU-48V network tap on eBay for use on my home net. This is one of their earlier models and has been discontinued for a number of years. They have two versions (AC or DC power input) of this model and the one I bought requires -48VDC power.

Per the spec sheet (http://www.network-taps.eu/products/prod_data/125/IG%20-%20TP-GCU.pdf), the two version (AC and DC) use the following power.

AC Power Supply:
Input: 100-240VAC, 1.5A, 47-63Hz
Output: 12V 5A 

DC Power Supply:
Input: -48V DC
-36V DC min,
-75V DC max

Terminal Block Pinouts
   Left: Ground
   Center: -48v
   Right: Positive

Now I can find 110VAC to 48VDC converters on eBay but none of them say -48VDC. So first question is, what exactly is -48VDC and how does it relate to 48VDC? Secondly, the spec sheet does not mention amperage for the DC power. The converters on eBay range from 200mA up to about 700mA. Do amps not matter with DC power and that's why the spec sheet doesn't mention it?

I've contacted NetOptics but haven't heard back so I'm hoping someone on here is familiar with either this piece of equipment or just power supplies in general and can give me an idea of the appropriate converter to use. Thanks.

Voltage is the electrical potential to push current through a circuit.

Polarity is determined by reference to circuit ground. In this case, the device has a positive circuit board ground plane and negative power in reference to that ground plane.

In circuits that expect negative voltage, you attach the positive terminal of your 48V DC power to the positive terminal and the negative terminal to the -48V voltage input.

Internally, the ground may also connect up to the positive device terminal which can be tested with an ohmmeter, or may be a chassis ground for shielding.