This shrub, native to the US Southwest, through Mexico and as far south as Nicaragua, is an important culinary herb in the cuisines of Mexico and Central America. It is also used in the US Southwest, but is still a bit difficult to find here in Los Angeles.
Some related herbs, particularly Lippia alba (White Lippia) and Lippia palmeri are similarly used.
More on Verbinas.
More on Herbs.
Buying: While possibly more available in Arizona or New Mexico, this herb is very difficult to find in Los Angeles, and probably impossible in most regions. It is, however, fairly available on-line, but shop for a good price. Always buy in leaf form and grind it yourself, as it is much more durable than pre-ground. Photo by Dick Culbert distributed under license Creative Commons Attribution 2.0 Generic.
Storing: In a sealed container with as little air space as practical, kept in a cool place and away from direct sunlight, this herb should last at least a year, in leaf form. Half that or less if powdered.
Substitute: To me, the Mexican oregano tastes more minty than Mediterranean oregano, so perhaps just a touch of dried mint leaf with the oregano leaf.