You may have to go outside the city, to a rural area. Try one of those farm supply stores TSC, I think... or a small town Home Hardware... my bro puts it in the outhouse at his camp, and it is hard to find, but that's where he picks it up...
I use dolopril and add it a few months before planting, a heaping tablespoon full should be fine. If your a nerd you could try adding more or less and testing the ph of the run off in different pots.
My advice if you are new to this I would just use a simple base mix like sunshine or promix add some worm castings and water with a simple organic liquid fert like earth juice or botanicare pure blend or mix up a batch of super soil.
Make sure the genetics you have will finnish on time and you will be all good, if you have a good soil mix you can pretty much just add water and nature does the rest for you. Goodluck