Chris Savery kindly recommended this place to me in another thread and even hand-drew a little map thanks to which I actually found it.
It's a tall, narrow run-down mall where every other shop sells second-hand cameras--the rest being toys for adults (not adult toys, you can't get
those in Thailand): toy models, "Magic: The Gathering" playing cards, dolls of SWAT team units (no joke) etc.
The most visible shop (it's on two floors), and maybe the best stocked one as well, is Takeshi Camera--again a place I'd heard of but had no idea how to find. I collected a few business cards so here are some phone numbers and most importantly, a bilingual map!
A lot of the stuff at Pirom is pretty antique but not all of it. They even have digital-only (cropped-sensor) lenses.
Chusak Camera: 01-9345654, 06-5134760
Panida Photoshop: 0-7012-4140, 0-2222-8700 ext. 201, 203
Camera Takeshi: 0-2222-8700 ext.241, 0-2222-8143 ext.217, 0-2623-7602 direct
contact@cameratakeshi.com and even a website (only in Thai):
http://www.cameratakeshi.com/