I've bought reps from BKK for years now.
I found the best plave is MBK. there are plenty of spots on the second and third floor that have cheap reps; rolex, B&O, diesels, g-shocks, and panerais.
The BEST place is a little stall on the first floor on the same side that KFC is on a couple stalls down from the currency exchange booth.
He has some of the best reps I've seen. AP, Patek, Panerai, rolex, IWC, JLC, Cartier, etc...
He's by now means cheap, but less expensive than I've seen on international sites.
I've become a "great customer" of his and always hooks me up with "Thai price".
If there is something that you don't see in the case, watch his magic and pull them out of fake kleenex boxes, behind calendars, and nehind your ear. Great guy to deal with and you'll get the best of the best.