The problem here is that while we have definitive evidence that abortion was known and practiced among the Aztecs, the details are a bit hazy. Women's issue, in general, tend to get short shrift in historical works. Something like abortion, which was a crime among both the Aztecs and the Spanish, doubly so. That most of the early Spanish chroniclers were friars does not help either. So I could rattle off a dozens of different herbs that Sahagun's General History of the Things of New Spain says can help with everything from gout to fever to diarrhea to UTIs, but the primary sources are mum on the specifics of abortifacients.

What we do have though, is some vague mentions that midwives would use "herbs" to induce abortions. There are a several plants with abortifacient properties in the Aztec pharmacopoeia, though recorded for other properties. Some of these are still used in Mexican folk medicine to terminate pregnancies, such as avocado leaf or aztec sweet herb. What has been most focused on though, is cihuapatli, which literally translates as "women's medicine."

The focus on cihuapatli is because it is explicitly named as an obstetric medicine. Its primary indication was to induce/hasten labor in full term women. As the Badianus Manuscript -- a collection of herbal remedies published in 1552 -- puts it:

If the woman suffers difficulty in the bringing forth, then that she may give forth the feotus with little effort, let her drink medicines from the bark of the tree quauhalahuac and the plant cihuapatli, the small stone eztetl, and the tail of the small animal called tlaquatzin.

Quauhalahuac is not identified and eztetl stones were put in just about every concoction. The cihuapatli, however, was tested in modern times an found to have oxytocic effects. If you're not up on your obstetrics, oxytocin is the key hormone for regulating labor, as it leads to cervical ripening and uterine contractions which together expel the fetus. Though ostensibly used only for labor difficulties, its mechanism of action could also work to terminate a pregnancy. Medical abortions today use misoprostol for the same oxytocic effect. As for the possum tails, Ortiz de Montellano (1990) posits that they may have an unusually high concentration of prostaglandin, which similarly has oxytocic effects. Nature is weird.

As for how to procure such a thing, we return to the aforementioned midwives. They were a recognized profession and a type of ticitl (physician). More than just attending the birth, they were involved from the very start of the pregnancy doing everything from warding against bad omen and behaviors (like chewing chicle) to providing massages to administering medicines for anything that might ail the pregnant woman. If anyone would have access to abortifacients, it would be the midwives.

Again, though, abortion was highly illegal among the Aztecs. Somewhat ironically, having an abortion or helping someone have an abortion could be a capital offense. A woman's role as a mother was a central to Aztec life as a man's role as a soldier. It is not a coincidence that men who died in battle or were sacrificed, and women who died in labor shared the same afterlife. Because of this prohibition against abortion, and historical bias, our scenario for how specifically an abortion would be carried out would have to be hypothetical. We could speculate that a rich woman living in Tenochtitlan might have more opportunity to employ midwives and greater access to specific herbs due to the central role of Tenochtitlan as a trade hub, but this would just be speculation. We do know that specific plants were normally used in a way that could produce abortions, and that there were other plants in use that we know have abortifacient effects, but the specifics are harder to come by.