Different ITX cases are designed with different small form factor power supplies in mind. You could be looking at pico atx, uATX, 1u or SFX power supplies. Personally, I have a Silverstone SG05B mini-ITX case and it's powered by a Silverstone 450W SFX power supply (it's basically a miniature version of an ATX power supply).http://silverstonetek.com/product_power.php?tno=7&area=en
It would be helpful if you could post what the case this is for and the specs of the machine you're hoping to power. My 450W supply is running an i5-3570K @4GHz with 8GB G.Skill 2133MHz ram and a GTX680.