Codeine is rarely sold without being mixed with another medication, such as tylenol or cough medication.
Oxycontin does not contain opium. Neither does fentanyl. Also hydrocodone (synthetic codeine) does not contain opium.
My hunch is we get most of our heroin from the Western Hemisphere. Opium is super easy to grow and will grow just about anywhere. You can purchase the seed in small amount legally here in the US.
Poppy cultivation and heroin production have been decreasing steadily in Colombia since 2001, when cultivation reached a peak of 6,540 hectares. By 2009, it had dropped to 1,100 hectares, the lowest level of poppy cultivation recorded in Colombia, according to U.S. Government sources (no estimate was made in 2010 due to bad weather). The potential production of heroin dropped from 11.4 metric tons of pure heroin to 2.1 metric tons, in the same period. In contrast, opium poppy cultivation in Mexico remains high, and Mexico continues as the primary supplier of heroin to the United States. Estimated cultivation of opium poppy reached 10,500 hectares in 2012, with an estimated pure potential production of 26 metric tons.
https://obamawhitehouse.archives.gov/ondcp/global-heroin-market
Where opium grows in the US:
500 opium poppy seeds for $5.99 on Amazon
https://amzn.to/35HJ1JO