I'm planning to do a mailshot to architects and designers in Thailand. Does anyone know of any ad/marketing agencies that sell targeted mailing lists?

Most agencies will sell or rent you a list of names but I'm pretty sure you'd not be able to drill down to that specific sector. Why not start with a professional organization or trade body? They'll have lists for sure and might be willing to share details with you. Another idea might be to do a deal with a magazine that targets that sector.

Good luck.