Take subway and go to last station named Bang Sue. Get in a taxi and ask driver take you to 281/11 Soi Sriphuen Rimklongprapa Fungsai Road, Ladyao. It's small house at end of dead end soi. Alternatively call 02-587-7704-6 and pass phone to driver so that they explain the address. Get there and try convincing driver to wait for you or else you will have to walk a bit on way out until you get to a big street. Go inside, office is small house on left side, and ask them for issue you want. They will bring it to you. Price is about a couple hundred baht.
