Wandering around Bangkok one afternoon in Mid-March, I spotted a vendor selling a number of grilled items, including small skewers of the chicken's triangular tail section.
I'd asked a nice lady at the hotel desk to write down for me, in Thai, the request: "Hello, how much is your delicious food." I presented this paper to the young woman behind the grill. She giggled and said "Five baht."
With baht at 30 to $1 US, I got two three-piece skewers of "chicken butts." That came to under 20 centers a skewer, which seemed a like a good deal, and about the right price for what I'm sure is considered offal, less desirable animal parts.
The young woman grilled two skewers for me and put them in a plastic bag, which seemed at first seemed odd (the bag crimpled up at first from the heat and started looking a bit melty). However, this turned out to be a good delivery system because the it enabled me to ladle in some counter-side hot sauce and use the bag as a dipping pouch.
The butt was surprising, I mean really surprisingly good. At home, this part of the chicken usually goes into the stock pot, but there is a lot of fatty flavor in these disregarded parts, and balanced with hot sauce, they prove an excellent street snack. Each lightly caramelized section must be eaten slowly, however, as there's a chunk of gristle in each that you have to nibble around.
During the ride into the city from the dock where my ship, the Zaandam, was ported, I had asked several people where I might find good street food in Bangkok, but that was a silly question: it's everywhere.