A lot of this stuff is being manufactured in the far east, primarily China, and enters our country through our “secure” southern border. As Obama opened the immigration floodgates, the opium, heroin, and synthetics followed.
Possibly...
Chemically speaking it's not that hard to make carfentanil from fentanyl, IMHO. (I'm a chemist.)
If you can make meth in a trailer park...