i dont think you'd need special food just because you do P90x.
my tip is: treat P90x as a normal HIIT workout, its not different from other types of circuit training (i know it claims to be special with its "muscle confusion" theory and all, but really.. its not. you can tell how real it is from the amount of money it has made with its "secret program").
as for your diet, get on a calorie deficit. as to how you do it, i suggest you log your food as accurately as possible (with either MFP or Loseit). i also suggest you don't eat what you can't measure. along with that, investing on a kitchen scale would be wise.
and finally, do not go crazy with your "diet" ... in most cases, you cannot willpower yourself out against your body, and you will end up binging. instead, eat smart, do not go hungry and eat whatever you want as long as it fits your calorie budget. the key to a succesful diet is based on its level of compliance.
