Any tile distributor should carry sulfamic acid crystals.
You dilute them in water as directed.

I believe even home depot carry's them in the same
section of the tile dept. that has grout sealers etc.

Sulfamic acid acts in much the same way as muratic,
attacking cement based products, but is milder and
safer. Muratic acid is like asbestos insulation...
there are too many safer alternatives to warrant it's use
in the tile business.

thetiler