But do be sure to take a copy of the Thai version of the map, in case you need a taxi driver to get you there.
I remember when I was in Thailand 10 years ago, the few bike shops that existed only stocked the smaller sizes - there was nothing even close to my 6'-2" height (188 cm). I doubt the situation is different today.
I know this isn't ideal, but there is a great shop in Singapore called Speedmatrix that sells Dahon folders and some recumbent bikes. I only mention this because I know that JetStar Asia has ridiculously cheap (read: USD50 round trip) flights from Bangkok to Singapore -- so if you're really desperate for a folder while you're there, you can always make a side-trip to Singapore! If you're in Phuket, I think the JetStar flights are even cheaper from there.
A '97 Cannondale F900 Killer V, a Bridgestone mini velo, a Strida, and a Dahon Piccolo.
I recently returned from Thailand and had my eye out for folders. Probike does sell the four Dahon models listed in their website, though their mainstay is the Trek family of conventional bikes. You may be able to negotiate the price a bit. They have a very nice shop just across the street from Lumphini Park.
Chinatown has one street, Luang Road (near Worachak Road), that has quite a few small bike shops. Here, I spotted a Strida hanging from the ceiling in one shop, and across the street was another shop selling a line of Chinese-made, but fairly good quality aluminum-framed bikes, both front and full suspension.
In some department stores such as The Mall and Robinson, I saw very cheap aluminum-framed, Chinese-made bikes going for around 6000 baht or so (around US$160). They were actually quite light and had Shimano 6-speed derailleur systems.
There are quite a few quality bike shops that are associated with the big brand names, and these will probably not have folding bikes but can be a good source for components. This site lists some: http://www.kutu.com/thai/bike/bybike.htm There are others, but I need to ask my brother-in-law for their addresses.
Yes, Singapore is the place to go for bikes and parts. My brother-in-law knows most of the bike shop owners in Bangkok, and they are always going to Singapore to buy bike parts and accessories. However, I've seen prices in some Bangkok shops that are cheaper than the cheapest mail-order places in the USA (I bought the entire drivetrain for my MTB in Bangkok a few years back).
Thanks much everyone! I'll head to Singapore. (China might be a bit out of the way.)
Have a great trip! Singapore is a great place for a relaxing break, though it will be more expensive than Thailand. To me, the whole place feels like it's on holiday all the time! (Of course, I'm used to living in Beijing and Hong Kong -- other people might find that Singapore moves at a crazy pace)