In popular history I mostly hear about obsidian used by the maya to create weapons for elite warriors, but obsidian seems like it would be extremely useful for many practical applications in an agrarian society like that of the maya. So would a farmer or other commoner have access to obsidian? And if so what might they use it for? How do we know?
I would not say virtually everywhere as /u/AlotOfReading has suggested because for example there are not known sources of obsidian in all of the modern state of Oaxaca. But this is no to say that people inhabiting those areas did not use it, since most of their obsidian came from particularly three sources: Pachuca, Pico de Orizaba, and Otumba. Obsidian was fairly used by every level of society but at different rates and depending on the area. For example, the commoners at Teotihuacan had very high quantities of green Pachuca obsidian because it was fairly widespread throughout the city. In contrast, the commoners at Monte Alban relay more heavily on chert sources within the valley of Oaxaca to procure their cutting utensils. But even they have small quantities of obsidian. Some sources were more heavily worked than others, for example the green Pachuca and Otumba in the central highlands; the Pico de Orizaba in the gulf area; and the Ixtepeje and Tajumulco in the Maya highland (which were some of the most widespread obsidian all over the Maya area. Maya farmers did have access to obsidian, but they also used resources that were perhaps cheater to acquired because of their closer proximity, like the honey chert from Colha.
One type of obsidian that did change in significance over the course of Maya history (and here I am just refereeing to the Mayas in particular no all of the other societies throughout Mesoamerica) was the Pachuca green obsidian. Early on was mainly used by elites and for ritual purposes, but then as it became more ubiquitous, it made its way into the lower classes and for more utilitarian purposes as well.
Obsidian was used nearly everywhere in Mexico. Due to Mexico's high volcanism, it's available virtually everywhere and makes very sharp edges. That edge was its primary utility for most people, being an excellent and cheap knife making material. Common people had ready access to the production of these knives (called prismatic blades) and they're found at every archaeological site. Obsidian was also used to make decorative items and jewelry and common people would have had less (but still some) access to these due to their expense.