I live in Bangkok and there are a number of good headphone shops here. You can find Yuin, Audio-gd, NuForce, and many nice local-made brands here.
You may browse their websites to see what kind of products you can find there.Sanook Gadget
Located in Central World
. You can find many "upper-end" products here, but they may not carry many "Far-East products" you're looking for.Munkokggadget
in Pantip Plaza (Computer Superstore). Here you can find many china-made products like Yuin, NuForce, HiFiMan, DarkVoice, and iBasso. The price may be slightly more expensive than ordering online, but you don't have to wait for shipping and can listen to the amp before you buy. They're also an authorized retailer of Grado.Hifi-Lover
Just on the opposite side of Munkokggadget. I know you're asking about portable amps, but in case you're interested in buying Audio-gd products, this shop is an authorized retailer of Audio-gd and has almost all their major products like DAC 19 DSP (which will be available for sure at the time of your visit), C-2C, Phoenix, and Reference One.Zeazon Mall
They're authorized retailer of Burson Audio and also carry many good portable amps.Discovery Hifi
This one may not be a headphones-oriented shop but they carry many nice DACs and accessories like HiFace.
If you're interested in particulars products and want to check the availability or price, just shoot me a PM. I may be able to help.