While the idea of stumbling upon a police warehouse full of drugs (some of which would probably be damn good quality too) is droolworthy, The Deceased has a good point that it would probably be pretty unrealistic, and the story would have to change a fair amount to accommodate.
Another thing you could go with that would still keep your idea's intact, plus give you plenty of different options is that he could stumble upon a dealers safehouse.
You still have a house full of drugs that can be taken, and it could just be one drug, or a number of different drugs, of any quality you choose.
The location can be anywhere and still be reasonably believable as a safehouse could be deep in the woods, in the country, really any remote location could be used.
Security also has a number of options too: It could be a high up cartel leader with lots of protection, a lower down dealer without anybody there at all, or anywhere in between.
I'm sure if you want to stick with it, you could find a way to make the police warehouse idea work, this is just another option.