Found this bit online for you. Hardware store is an idea.
" Epsom salt is magnesium sulfate. This chemical is sold under several names, including 'sulfato de magnesia', 'sal de epsom', 'sal inglesa' and 'sal de higuera', and probably others.
In Mexico, pharmaceutical grade Epsom salt (sulfato de magnesia) for internal use that you might find at a farmácia is quite expensive.
For a foot soak, I would search around at hardware stores to see if you could find a commercial grade in bulk that would be cheaper."