You can bake it yourself. Just buy a 1lb container of arm and hammer or whatever brand you like. Put it on into a ceramic bowl or foil and bake it in the oven for a good 30mins-1hour at like 300F or so.
HOWEVER baking soda and other alkalinity buffers should not be used as a pH controller. Baking soda influences pH by increasing alkalinity (your waters buffering capacity) allowing it to neutralize more acids in the water and thus raising the pH. However you only want to raise your alkalinity to a certain level, NSW is around 8 dkh (I keep mine at around 9-10).
CO2 in your water is gonna be the 2nd biggest factor, high co2 will lower your waters pH level. Increasing alkalinity a buttload when there is an excessive amount of CO2 in the water won't do anything and can be even more detrimental. To address that you need to increase oxygenation by aeration (possibly from an external source if your house air is high in co2) or by use of a refugium or other algae.
Nitrate issue
Using appropriate water with your water changes can help a lot. If you use bucket salt with ro/di water it'll be a lot cleaner than using catalina water. This will help with your alkalinity issue as well, however most companies tend to keep it near NSW levels of around 8-8.5dkh. If your tank is say at 6dkh and you do a 50% water change with 8dkh water then your tank will be at 7 roughly. So dosing in this case is necessary.