Ask for the extended menu and you'll be fine. You can also try the vendors that offer their stuff at every station where the train stops. If you're using the ferry, don't buy a ticket in advance, because the further away from the pier you buy your ticket, the more expensive it gets. If a policeman catches you smoking pot you will be fined 50,000 baht, get kicked out the country and receive a stamp in your passport so that you'll never be able to enter the country again. And I am not even talking about spending a couple of days, or even weeks, in a Thai prison.
Thai police are pretty harsh on dope-using tourists.
The police even use tourists as moles. This is the downside. The upside is that it is pretty easy to get the herb. But always keep the above in mind. If you want to smoke without hassles, you should go to Amsterdam.