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).
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.