As long as you're not stupid, Laos is a great place to buy and use opium. It's easily scored in Luang Prabang and Vang Vieng. Probably/certainly other places too, but I'm only giving you my direct experience. I guess the older you are, the less likely you are to get a police search. If you do get caught with personal amounts it's a $500 fine/bribe to get yourself out of it. I think you have to be particularly stupid to get caught though. Although everybody can get unlucky I suppose. But the first time I scored was directly opposite Luang Prabang police station. I've never had hassle. Tuk-tuk drivers are your friends. You need to bargain, they will start at stupid western price. If your bargaining is good you should reduce this price by up to 90%.
Cambodia is possibly the best country in the world to score a wide range of drugs. I never got opium there but it's certainly possible. I definetly think you get the best weed in SE Asia in Cambodia. Their pharmacies also sell the best things OTC that you can get anywhere. This may just relate to PP though. Ask Jude, he lives there.
Laos is my favourite place in the world. I'll be there in about ten weeks from now.