Tunze power supplies tend to be pricier because they are switchable between 12v, 14v, 18v and 24vdc. Well, that and that they are higher amperage (most of the smaller switchable wall-wart style power supplies are usually in the order of a few hundred milliamps). I forget what Tunzes need offhand but you can generally figure it out by looking at the wattage they use (watts = amps x volts).
That said, usually we don't switch the Tunzes around after settling on a speed, so if you don't need switchable that might open up some options.
18v is pretty common but some people are pushing them at 24v now. Either way is OK according to Tunze, depends on how much speed you want out of them. I find at 24v they are louder but some people don't seem to mind.
So pick a voltage and see what you can find. Any 2.0 amp, 24VDC or 18VDC power supply will do. Heck, if you have an old laptop power supply it is probably 24VDC 2.0amps. But you can get these at electronic stores. For example,
http://www.memoryexpress.com/Products/MX25408
Worst case scenario you might have to cut and crimp the old connector onto the new.
Hope this helps..