Gee, thanks, Archimedes!
Dry fertilizers are available in 50 lb plus bags to the aqriculture industry. It took our club (SFBAAPS) several years to use 50 lb bags each of several agricultural grade fertilizers.
Several aquarium minded folk buy these bags, then split them up for sale. Usually 1 lb quantities.
Here is one such place. Look also in the sales area here and other sites.
How to estimate how much to buy:
If you need to dose KNO3 (to get the nitrogen) and KH2PO4 (to get the phosphate) then you might be dosing enough K at the same time. Maybe, maybe not.
If your tap water GH is over 3 degrees I would skip the GH booster, unless you know your Ca or Mg was missing. GH measures both together. To find out if your water is weird this way, test the Ca. Sorry, you will have to put on a thinking cap next, it is not a straight math formula to find out how much of your GH is Ca, when you have done a Ca test. If you are going down this trail, know that there are ways to dose just Mg (Epsom salt) if that is what your water is missing, and ways to dose just Ca if that is what your water is missing (Calcium carbonate is one, there are others)
If you have a heavily stocked tank and the NO3 keeps rising, then you may not need to dose KNO3 or KH2PO4, so get some K2SO4. This is potassium without the nitrogen or phosphorus.
So: Here is the shopping list (2 of them, actually):
Better list, especially if you have good light:
KNO3 (if fish food does not supply enough)
KH2PO4 (If fish food does not supply enough)
K2SO4 (Get some anyway, even if you will be dosing the first 2. Excess potassium does not seem to be a problem in aquariums)
GH Booster, or specialty Ca or Mg products, only if you need them (Epsom salt can be purchased very cheap in stores near you, do not pay shipping on this)
Trace minerals. I would get some, even if fish food supplies a fair amount.
Iron: I would get a chelated iron. Even if your tap water has iron, it may not be a form plants can use.
Keep up with Excel as a source of carbon, though there are other methods. You can make a CO2 generator with yeast and sugar, and a little minimal plumbing skill (Very minimal) or you can get fancy with pressurized CO2, which does call for a bit more skill in set up, but is well worth it if you can afford the initial expense.