Do you have a DVOM? (Digital Volt Ohm Meter) If so, check the voltage coming off of that white wire. (very carefully.. we don't want any electrocuted people here


If its 12v DC then you should be fine going with a computer fan. If its AC or a different voltage, you may be in a bit of a pickle. If AC, you'd need to find an AC fan to replace it, though I seriously doubt they'd use an AC motor in that fan. If its lower than 12v, you could still use a pc fan, it will just turn slower than it is rated to, and therefore move less air than rated for. If more than 12v, which I also doubt, you may damage a pc fan or have it burn out relatively quickly, and you may notice that it will be louder than rated for, since the fan will be in overdrive.
Is there any way to contact the manufacturer and order a replacement fan from them? If so, that'd be what I'd do. If not, check that voltage, and see if you can find an adequate replacement from Fry's Electronics, or something of the such. Good luck to you.
Oh, final thought: you shouldn't need to break open the new fan. Splice the wire you have coming out of the fixture for the current fan, and attach those wires to the wires coming out of the new fan. If its ~12v, the wiring on the new fan will be fine to handle that... either solder the connections and wrap with electrical tape or use those twist-on connectors and tape those on.