Mexico prohibits the import of seeds and plants from other countries. Which means Amazon and Burpee Seeds do NOT ship to Mexico. Baker Creek Heirloom Seeds does but customs can sometimes hold the shipment up for months. Other online places to find seeds in Mexico are La Semillería and Rancho Los Molinos.
My favorite place to buy plants is at the weekly tianguis (fleamarket). Usually, I can score a coffee tin or two with plants for under 20 pesos. You can also buy tierra para plantas (feedbags full of soil) from certain vendors. This is usually dirt scooped up from the base of el acebuche (Olea europaea), La ensina (Quercus ilex) or uña de gato (Uncaria tomentosa)trees which is particularly porous and makes a good mix on top of the regular old black dirt. This is what Mama Sofia collected and sold to supplement her income in Cerano.
Seeds packets are most often found at places that sell animal feed, maicería or Alimentos para animales or forraje. The packaged variety is limited but every now and then you can find exactly what you were looking for. I spent 6 solid months looking for semillas de jamaica (hibiscus flower seeds) until I finally found some. They’ve sprouted and I’m excited to see how they grow! These are the best places to get your corn, pumpkin and bean seeds by the kilo or puño (fistful).
You might also try stores that sell productos de jardinería (gardening products). You’ll find macetas (flowerpots) and some general insecticides here. Macetas are also sometimes sold off the back of trucks that periodically come through town offering 3 for $100 sets.
Viveros (plant nurseries) sell live plants but it’s a pot shot what you might find. The other day we were hot to get a banana tree and went to 4 different viveros before we found one. We also lucked out and found a new barrel cactus to replace the one that putrified in the last rainy season.
Sometimes you might come across a tricycle vendor and I encourage you to stop and take a look. You might just find exactly the plant you are looking for.