According to the 1971 DoD report marijuana was readily available and easy to purchase locally. 92% of servicemen said that it was “always or usually available”.

John Steinbeck IV (son of the famous author) wrote an article in 1968 called “The Importance of Being Stoned in Vietnam”. In the article, he states “For the most part, the drug [marijuana] is cheaper and easier to find in Vietnam than a package of Lucky Strikes” and “In any case, marijuana is as easy to find in Vietnam as a Vietnamese national, and you needn’t find one to find the other.” He also says “One has merely to stumble on his way outside an American billet to bump into a man who might be selling the plant. Or he may take a deep breath anywhere to smell its dusty aroma. It is the slightest of achievements for an American to “score” on more marijuana than he has ever seen in his life.”

So, suffice it to say that yes for marijuana you could basically just walk down the street and buy it.

As to heroin that’s a different and more interesting question.

I’ll note here that many scholars and writers (including Steinbeck in that article), as well as the DoD report of 1971 and a later one from 1973 all argue a similar theme. The military cracked down on marijuana usage arresting hundreds of servicemen a week, and subsequently led soldiers to use heroin. This assertion can be seen in the report and article linked previously as well as argued here and here.

Without marijuana, soldiers seemed to turn to heroin as it was odorless, easy to conceal, and could be consumed in a variety of ways. Like marijuana, it was also pretty easy to get and fairly prevalent among troops but there isn’t as much data on where people bought it or found it. One interesting piece of data is that 2/3 of those that tried heroin or opium while in Vietnam did so within the first two months of being there (see DoD study linked above). This likely means, as opposed to marijuana which was bought from locals, soldiers were obtaining and using hard drugs at American bases, posts, and training grounds before ever being assigned to a combat or foreign position.