Today I had someone email me looking for a source to obtain Tequezquite, turns out that if you do a Google search, this thread is the first thing you see.
Anway, someone that I know did some checking and came up with this information. In the future we may be able to offer this for sale, but it seems it would have to be brought back into the states from Mexico.
TEQUESQUITE, TEQUIXQUITL. Sodium chloride and carbonate
Saltpeter or efflorescent salt appears in dry seasons on abandoned ground by lake waters of the Mexican Valley. The natives of Ixtapalapa controlled this salt's business, who according to its quality, classified it in espumilla, confitillo, cascarilla and polvillo. Poor natives used tequesquite instead of salt and some of them still consider it essential in the preparation of some dishes, especially to soften beans and cook nopales and vegetables, so they keep their natural color.
Tequesquite can be used as a yeast. This condiment is prepared for thatmatter: ten transparent green tomato skins and one stone of tequezquite are boiled in one cup of water; once the stone has been dissolved and the water has boiled once, it is retired from the fire and allowed to settle. When it is cold it is strained and is incorporated to any dough to be fluffed up.
To make vinegar, tequesquite is added to pulque (fermented juice of maguey plant), putting it on the fire but making sure it does not boil. It is taken off from the fire and is left to be fermented for two or three days.
http://www.uv.mx/popularte/ingles/scriptphplen.php?&sid=246&len=In
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Tequesquite is an alkaline mineral deposit that is used when making tamales and bunuelos. (Some people boilit with tomatillo husks and use the water when mixing both doughs to obtain the same effect.) Tequesquite is widely available in spice counters of most Mexican grocery stores and through mail order.(See Sources on the web site.)
http://www.zarela.com/ask/archive/Tequesquite.html
http://www.zarela.com/resources.html