First get a quality test kit. Salifert is good for Alkalinity testing. Then practice. Do a couple tests in a row and see if you get consistent reading. Practice this every couple days till you feel comfortable and get repeatable results, within reason.
Example, 8.2 and 8.4 close enough I’d feel that I’m doing test correctly. Majority of reefers keep Alk around 8.0 dkh
For the supplement? Do shop online or does your LFS carry what you need? Powder or liquid? Sounds like you have a good LFS, see what they have and use for their tanks. Basically, baking soda is what is used to raise Alkalinity.