Wherever it's coming from, it's coming in through ports of entry.
Americans are bringing it in.
I did a research project one time and found that 25 square miles of poppies would supply the entire US dope habit. It may seem like a lot, but it really isn't. (The US comprises ~3.8 million square miles) Fentanyl has a similar dynamic, except that it's stronger and takes less room. Now, understand the difference between a pull economy and a push economy. The demand isn't there because the drugs are available. The drugs are available because there's a demand. If you want to combat drug abuse, treat the demand. You will never stem the supply.
The wall is a trillion dollar boondoggle, an enormous pool of money for the greediest to dip their hands into.
Trump's wall can be defeated with a cheap reciprocating saw from Home Depot, a ladder made of rebar or a flash flood in the arroyos. It's fucking stupid.