Laws for tourists can get expensive/bad! Be careful its a funny culture of needy desperate ppl.. Its no joke that a guy will sell you stuff on the street for a decent price, then tell a cop what he did and point you out for a few bucks.. If he does that then you'll need to bribe the cop alot more than you payed, and he might haul you in anyways.. Citizenship has its advantages..
Your safer sources will actually probably be on your Americanized resort area, but the prices will likely be much higher than what some rasta dude on the street has to offer..
I've never been to Jamaica, but speaking from experience :
About 3-4 years back my brother and friends bought some weed, was lower than low quality, for $5/g... His buddies also bought some coke, half of it was salt and resulted in nausea and vomiting... He bought this shit from vendors on the beach
Lesson Learned - People from foreign countries will rip you off, and the stuff you buy probably wont be that great or 100% of what you intended to get
Now this was Cabo San Lucas, the people there are poor and the tourists are rich, so I can see why they would rip people off...
As for Jamaica they might be a bit cooler about it, but it's really probably hit or miss...
If your not going to smoke much invest in one of those produce packagers that hermetically seals stuff, and seal a small amount and keep it on your person... Actually I wonder if that would work, I'll have to make a thread..