You can find salt at your LFS: Good brands include SeaChem, and Red Sea: both good quality aquarium salt. If you must look online these are again the two good brands to look for. Remember not to add salt willy nilly and use a hydrometer/refractometer to measure the salinty so you dont overdo it and produce an impromptu marine. Also remember if there are fish in the tank you will need to acclimate them to the salt: either add it slowly with water changes and bring up the concerntration with time, or rebag your fish and acclimate them as if you were putting them into a tank from the LFS.