Hi Kleeem,
I personally do not recommend the cited software called HydroBuddy from Science in Hydroponics for a number of reasons. It has become a nerd tool, is over-engineered and yet doesn't calculate what I am asking for. It is way too complicated and not what I'd recommend for a beginner or an intermediate
nutrient maker. Also, the firstly enthusiastic development has stagnated for a while now and for some unknown reason, the author is only sporadically available for questions and recommendations. The author is indeed a qualified chemist, - but apparently not very experienced in practical nutrient making. You can always give it a shot, - but as someone who is doing
nutrients professionally for many years, I have warned you.
As for resources of specific components like diammonium phosphate, (plus a calculator) I recommend a supplier called
customhydronutrients.com.
I am in close contact with the owner who is a very knowledgeable person in my mind and I will send you a PM in order you can give my reference. If they don't have what you need, they can always consult you and/or point you to a provider near your location.
The
Nutrient Calculator they provide for free, is transparent and fully editable and uses the exact guaranteed analyses of the components (they offer) and not the one of lab grade chemicals. It allows you to actually understand how nutrient calculations are done - and this is the best way to actually get what you want and have most accurate results.