Ace hardware has it. It is hydrochloric acid. It is very safe for your reef if you have some residue left over, but not so much if you get it on you, so just be careful. Most pool/spa places will likely have it.
A gallon is like $8 and it goes a lot farther than vinegar. Once diluted with water, it becomes completely benign - you can add it directly to your tank if you need to lower the PH (all kalk users should have some on hand for emergencies). I just glug in some to a brute really scientifically and soak stuff in it and then pour it into the rocks around my house. I don't know for sure, but I think that a gallon of Muratic would be equivalent to 8-10 gallons of vinegar.
Unlike vinegar, it will not add carbon source to your tank if you don't rinse well enough.