Help me find a replacement external OEM PSU

What's it used for? I can't tell from your photos. Try searching on ebay or do a google search. You can usually find aftermarket PSUs like the one shown in your photo on ebay for much less than what the manufacturer would charge.

The power connector appears to be a standard 4-pin mini-DIN connector, similar to a S-Video connector without the keying tab in the center. Take a look at the first comnnector on the left on this wiki site:
http://en.wikipedia.org/wiki/Mini-DIN_connector

If the PSU identifies the pinouts, voltages, and amperages then you should be able to find a generic PSU similar to one used for a laptop and put it together yourself. Based on the fact that it has four pins instead of just two, I'm guessing it puts out two different voltages at different amperage ratings (probably 5VDC and 12VDC with ground pins for each). If you can't find a single PSU with dual voltages you could always use two separate PSUs tied together with one connector that plugs into the unit. It won't be as pretty, but it will be functional.

BTW, have you tried contacting Niveus to see what they'd charge for a replacement?