If you do buy Campden tabs please be sure to check their concentration, they do vary though the most common is potassium metabisulfite. The sodium version can be used in wine but most shy away from it due to its sodium component--but many do use it for a sanitizer. Plus, not all brands of Campden dissolve without leaving a sediment which is due to its compounding method. There is one that I have found to be totally sediment free, but I forget who makes it. The granule version of k-meta is available in bottles or bags. And be sure to read the instructions for use--different concentrations are out there. That rule actually applies for every additive out there(nutrient, energizer, enzymes, sorbate, fining agents, etc.). And do not forget that things expire, writing the date you open any container is a good practice to get in to. Nothing worse than finding out your sorbate 'died' and fermented out the honey you just added to backsweeten your mead.
If you want to take the guesswork out of SO2 additions you can buy a SO2 Test Kit. A great investment, and not too expensive either.