This academic paper outlines a study investigating the extent to which illegal manufactured fentanyl (IMF) supply varies over time and geography, as well as how the price changes based on these factors. The author's found that since 2013, the IMF problem has become more deadly and diverse.
Kilmer, B., Pardo, B., Pujol, T. A., & Caulkins, J. P. (2022). Rapid changes in illegally manufactured fentanyl products and prices in the United States. Addiction, 117(10), 2745–2749.