Locally you can probably find it at wine making shops. It is used as a yeast stimulator.
Why do you want to use Diammonium phosphate? No aquarium oriented shops will carry it. You'll notice if you search the forums here only 6 threads talk about it. 4 of them are in relationship to diy yeast co2 and 1 of them is your thread.... As a point of reference, there are 182,167 threads here!
I understand that there is more phosphate in Diammonium phosphate.... but I don't know what the point is, really. Some secret the rest of us don't know?