For the problem of needing a positive supply and only having a -48VDC supply available, I'm going to stick my hobbyist neck out and suggest this (if I'm wrong I hope I'm quickly corrected, but this is what I would try):
Voltages are relative, not absolute. You seem to know this since you mentioned specifically: ". . .-48V terminal w.r.t earth." So if you think about it another way, you could say that the one you're calling "earth" is actually +48V w.r.t. the one you're calling "-48V".
Measure the voltages across the two lines, but put your meter's ground lead on the line you're calling -48V and the +V lead on the line you're calling "earth". You'll see +48V.
In short, you can use those lines as your power supply, just use the "earth" as your +48V line and the "-48V" line as your ground. i.e. just swap the polarity.
Then you can use a DC-to-DC converter like tcmtech mentioned to step down efficiently to your required voltage.
Make sense?
Regards,
Torben