You don't understand the industry. You are thinking in the context of nice academic labs with thick budgets.
Where do you think the fentanyl people are getting their ingredients from? Asia. You can easily get most industrial/pharmaceutical chemistry pre-cursors and analogues/isomers for the semi-legal/regulated substances. Whether it can clear customs is another story but for kitchen chemistry the internet is more than sufficient.
Go to an aggregator like "Alibaba" and do a search. You will be surprised at what you can find.
In many parts of Europe - e.g., in Germany and in the UK - the purchase of many common "dangerous" reagents is regulated and illegal without a license that isn't available to hobbyists. The government takes the view that it's simply not a legitimate hobby.
In the US, the situation is better, but some essential chemicals are unobtainable due to DEA legislation (iodine, phosphorous). Many others are subject to DHS and DEA monitoring and a patchwork of state laws that prompted most reagent manufacturers to stop shipping to non-commercial customers altogether - and in recent years, also caused them to also crack down on resellers (requiring every reseller to attest that they will not re-ship to civilians). In the past five years or so, eBay also had major crackdowns, banning everything from sulfuric acid to potassium iodide.
And that's before we get into raids on YouTube chemists, etc. Heck, Texas prohibited people from buying or owning laboratory glassware until recently. Wikipedia has a good summary.