You should be able to find a tire in La Paz, maybe not the brand and type you want, but you should be able to find something that will work. A 120X18 shouldn't be too hard to find in most larger cities on the mainland as well. If a larger shop doesn't have one on hand, they can usually get one from DF on an overnight or 2 day bus trip. +2 on the central highlands of Mexico to get out of some heat. Copper Canyon, Durango, Real de Catorce, Zacatecas, Guanajuato, San Miguel de Allende, then toward Morelia/Patzcuaro, Oaxaca, San Cristobal de las Casas, all the way into Guatemala at La Mesilla, where you climb up into the highlands once again. San Miguel can tend to be pretty expensive since its a big gringo retirement community, but a beautiful town nonetheless. The coast will be absolutely brutal hot this time of year, but there are some sections of Mex 200 worth running like from about 60 miles south of Manzanillo to Rio Nexpa/ Lazaro Cardenas. If you go down that way, be sure to stop at Rio Nexpa, nice little chill surf community with a couple cheap accommodations, surfing and fishing. But man, its gonna be hot!