Yes, fans are DC.
As for the power supply, do I need to find one whose output exactly matches the input of one fan? What specs should the power supply have?
Think about a car battery. They are 12V and can supply upwards of 600-1000 amps when you start the engine. However, when you turn on your stereo or headlights (all run off that same 12 volt battery), they don't blow up because they are being force-fed 600 amps; devices will only draw as much current as needed, at some specified voltage. A lightbulb in your house is the same; the electrical grid can supply hundreds of amps to your house, but a 60 watt lightbulb only draws around 0.5 amps.
Take away: Once you have the proper voltage for a device, the load (a light, fan, etc) will only pull as much current as it needs.
So, the trick with power supplies is to remember that you first need to determine the operating voltage, then have your supply be able to produce enough (or more) current for the expected load.
If you have one fan, it will take 12V and 0.45 amps to run. If you connect two fans in parallel, they will each need 12V across the power wires, and each will draw 0.45 amps, so 0.45+0.45 = 0.9 Amps. If you need 3 fans, all three will need 12V across them, and each pulls 0.45 amps, so total current needed will be 3*0.45A = 1.35 amps.
Once you know the voltage and current, you can find a power transformer that will take 110 VAC from the outlet and output 12 VDC, and at least the amount of current you calculated. In the case of two fans (2*0.45 = 0.9 A), you could use a 12V supply that can supply 0.9A, 1.0A, 5.0A, 10A, 10000A.... anything, just as long as it's current capability is greater-than-or-equal-to the amount you calculated.
For 2 fans... 12V @ 0.9A, you can find something like these at most electronics stores (Wal-Mart, maybe... Radio Shack, definitely).
Google - 12V 1A power supply
Hope that helps you understand how voltage and current ratings work.