48V.
48V is an old telco standard. It was chosen as 48-ish-more-or-less volts because that's what you get if you connect four 12V lead-acid batteries in series, and back in the dark ages telephone exchanges used that as their UPS. 48V is what gives you a dial tone, and it's at the sweet spot in another way too: High enough to send power very long distances (ie, your phone) but low enough that it's not going to kill anyone*. It also, by fortunate coincidence, is just the right voltage for DC distribution in a data center. All that needed changing was to set a formal standard for acceptable deviation from 48V nominal.
48V is also the supply for PoE, and for phantom microphone power. For much the same reason: It's low enough to be safe and practical without requiring too much insulation, but high enough that current and drop are manageable.
Take a look on eBay: There are tons of 48V supplies.
*It hurts, though.
Biting the hand that feeds IT
