When I build my box, I was trying to figure out the same issue. Instead of going through all that work, I bought this power supply
http://www.newegg.com/Product/Product.aspx?Item=N82E16817152019
and these fans
http://www.newegg.com/Product/Product.aspx?Item=N82E16835185092
The power supply only cost $15, and you can essentially daisy chain as many fans together as you want as long as they pull less than 380 watts. I found this solution much easier than wiring individual plug in fans. The modifications you need to make to the power supply are much easier than if you bought a 12v adapter. Here is a link for instructions:
http://www.gideontech.com/content/articles/196/1
Basically, when you look at the plug that goes into the mobo you'll see 20-24 pins with different colored wires. The green wire is the "ON" wire, and every black wire is a Ground wire. All you have to do is (UNPLUGGED PLEASE!) get a paper clip and bend/cut it so that one ends is in the green plug, and the other end is in any black plug. The circuit is now complete and the power supply will automatically turn on once you flip the switch, no motherboard needed.
On a side note, if you are growing stealth like me (I grow in an apartment) noise is a big factor. Those fans above are hands down the QUIETEST fans I've ever seen. They move great air, and you literally cannot tell they are on once you turn them on.