Here's one of the all time street-food favorites: Roti 'pancakes'. It's one of those things you will ONLY find on the street, and never in any 'proper' restaurants.
Roti pancakes in one form or another are found all over South and South East Asia. I'm not sure where they originate, but it could very well be an Indian thing. (comments/suggestions welcome on this one). While roti in many countries are a savory bread/pancake eaten with spicy curries, the roti you find on street food stalls in Thailand are pretty much always the sweet variety: pancakes with sweet condensed milk-syrup, sometimes with egg and/or banana added. They're not really pancakes, the dough is formed into a very thin round shape, pretty much like Italian pizza. The dough is then fried on a hot plate until it's done and nice and a bit crispy. I've tried to capture this on 'film', using only my phone-camera so the picture quality is just a wee bit short of National Geographic standards.. : 8-) It does show the process though.
Plain roti's are typically sold for 5 baht, and up to 10 or 15 baht when adding egg and banana. Believe it or not, but I didn't eat roti for quite some time because I considered 5 baht to be overpriced... 0) Yes, really. This because I used to spend some time in Sukhothai, and there at the bridge over the Yom river was a truly excellent roti stall, run by a very nice lady who sold plain roti with sweet condensed milk for just 2 baht.. Then when going to Chiang Mai I found them at 5 or even an extortionate 7 baht and consequently considered this was WAY overpriced.. Fortunately for me I'm over this now. :-D Anyway, while in Sukhothai I became one of this Roti lady's best customers, and she even let me try to make roti myself at her stall, flipping the dough around to spread it out and create a thin pancake. Almost needless to say, I sucked at it. People, making roti is seriously difficult! Better to invest the 5 baht I'd say. ;-)
Location: Roti sellers are mostly of the nomadic kind, so I can't really give one fixed location to find this one, or even any one roti seller in the city area. So you will have to locate one yourself. :-)