Baja is different. Tires are widely available throughout Mexico. Most cities have at least a few bike shops and you can find something that will fit.
Price and selection are the wild cards. The BMW shop has been closed there for about 2 years now. Few businesses, like bike dealerships, stay open past 4pm on Saturdays.
Regarding your tires, of course it would have been easier if you had fitted new ones in the US before heading south. But if your tires got you down the Baja, they should get you to either Guadalajara or Mexico City and you'll find something there. Metzeler, Pirelli, and Michelin can usually be found in a city just by looking around. In many cities, the bike shops are conveniently located in a certain block or a few blocks of the city. You will often find a much better deal at the smaller independent shops. Dealerships charge a massive mark up on items (usually) especially BMW. The difference can often be $300 to $500 pesos or more.
Importing anything from the US takes time, and if the item carries any ID on it that says "Made in China" or a few other countries, you will be paying a big tariff on it and it can often lead to a longer wait. Bringing in tires from the US by a service like DHL or UPS (both are better here than FedEx) is going to be expensive, very expensive. I see a lot of riders coming through here (south east Mexico) with many different tires. Ask yourself seriously how much off roading or dirt road riding you are going to be doing. A dual purpose tire on a KLR is about all you will need unless you plan on riding your bike fully loaded off road, which most people don't do.
The only time I have felt limited by tires on my F650GS was trying to summit the Cofre de Perote on two occasions with Continental Trail Attacks and a Metzeler/Continental combination. It's the 8th highest peak in Mexico at over 14,000ft and you get there on a gravel and then broken stone/gravel road. Unless you are doing something like that, you should be fine with Continental Trail Attacks, Pirelli Scorpions, or any Metzeler or Michelin offerings. Bridgestones wear well, but suck off road. Heidineaus are expensive and difficult to source, most people don't bother with them.
If the tire holds on greasy, wet tarmac or not, that is the real question in Mexico.