If you cannot get a standardized solution of HCl, seek out "muriatic acid" at a hardware store. Muriatic acid is a hydrochloric acid solution with a concentration of approximately one part HCl to two parts water.
The muriatic acid will need to be diluted by roughly 1:60 to get an acceptable simulation of gastric juice.
Muriatic acid is used for cleaning bricks and can be found at most hardware stores.