It's common streetwise jargon. If you're a drug dealer, you shouldn't take the drugs you're dealing or you'll become addicted yourself and stop making lots of money. You're not supposed to get high on your own supply (of weed, coke, heroin, etc.) or you'll lose focus of your goal: making money at the expense of others.
Clearly no real drug dealers here. This expression is not about not taking the drugs you are selling. Back in the glory days of Revolution and recreational drug use, anyone who forwent the enjoyment of a commodity he expected others to get off on-- would lose all his customers, and soon.
This is/was especially true of weed. Smoking illicit herb is a consummately social act, and at least before prices increased almost tenfold during the eighties, solitary smokers were looked down on and mistrusted. This went double for dealers, even though anyone with common sense should know that "staying for the party" was a physical impossibility for retail-unit merchants with any real clientele. Even so, the token toke was a necessity, and a definite built-in hazard for any dealers who got ambitious or wanted to get rich. A drop-and-run attitude got you a rep as "Mr Businessman," and people sought more sociable sources-- such people as were thought to be more trustworthy.
"Getting high on your own supply" was strictly about gluttony, overdoing things, putting self-indulgence ahead of the service of others-- your first clue you were doing this was when you found yourself indulging alone, at home. Your second clue was when you had a meet and you'd gotten so stoned, at home with a prospective replacement for your ex old lady, that you couldn't remember whether to bring money or weight to it.
In my day people who got high on their own supply were a self-correcting problem to society, but there was definitely that ten percent or so who couldn't quit raiding the stash jar at all hours, and needed to find another line of work. Nowadays the drugs are presumably more powerful or irresistable-- it could be "getting high on your own supply" is a lot harder to avoid, and customers themselves are a lot more businesslike about scoring, and there's no reason to cut into your profits, except of course for quality control-- the test flight of each wholesale unit before you break it down and weigh it out.
Of course strong, microdose commodity like LSD was its own special case, and it may well be in psychedelics that the cautionary expression originated. The canard was, that you could absorb enough chemical while cutting up blotter or counting out microdots-- to end up tripping your brains out. There were people who avoided "getting high on their own supply" by wearing rubber gloves during this procedure. .