Hydrochloric acid is also the acid sold to doctor the pH of swimming pools.
Note that most etchants, including their FUMES, will happily convince many metals that are normally considered "stainless" (Stainless steel, chrome, copper...) to corrode. This is particularly a problem with the CuCl and HCl/H2O2 etchants, since they're normally used with an aereator that kicks up a lot of fumes and/or microdroplets 
1 part HCl added to 1 part "drugstore" peroxide seems like a good starting point. I dissolved a bunch of scrap copper in mine to get the CuCl2, but I gather that the acid/peroxide is a pretty good etchant all by itself.