Capital Hill is known for that... but if you go to the park or whatever, mostly what you find is annoying people trying to sell you crappy shwag... A lot of street kids also try to sell you stuff, but who knows what it is.
Downtown Denver is basically really slummy to me, just walking down colfax u can probably score, especailly at the bus stops, but again its just low quality shite. If you want to score some nice bud, you have to search for contacts, its hard sometimes and frustrating, but it pays off in the end.
You can always try west lakewood or Golden, just ask around at the bars, its in high quantity there.
But Im tellin ya man, if u live in fort fun, hang out near the campus and you will eventually find some, one time I even got some by Jim's Wings from some college kid, thats right off Elizabeth near campus.