Any hydro store will carry it... o/l or local. I use 1ml/gal routinely & it contributes appx 70ppm. You can go from there. GL. More info w/in prev posts i'm sure... some use crazy amounts of 4-5ml/gal. Peace.
coco is great , dont let your cal mag worries stop u from getting hooked on it, a good coco(im useing botanicare) and a food with cal mag in it and youll be happy with your results, im amazed how coco is so easy to work with, u can hear the water seep through threw it each time u water drawing in fresh air and making your girls happy!!!!
I am on well water, and when I was filtering it (480 ppm to 300 ppm) it seems that the main thing the Tallboy filter was stripping was cal/mag! I bypassed my filter and just top off my reservoirs with straight well water now, and the plants are much happier and I haven't had to add cal/mag at all lately. Just depends on your water source.