I am in Canada, so I can't help you with where to buy dry fertilizers (assuming you are in the U.S.). I am sure that others can help with that. I use small containers to store a portion of the fertilizer for easy use and pack away the larger bags containing my stock of fertilizer. I use baby food jars, but any jar would work.
As for dosing, how heavily planted is your tank, and with what kind of plants. Also, what kind of dosing schedule are you on now? Would it be possible to provide a picture?