There are a few places I recommend you.
1) Werng-Na-Korn-Ka-Sem: It's around China Town but not exact China Town.
The Stuff here your could get quite good deal are
Asia Music: Schecter, if you are lucky to meet, The owner's daughter is pretty.
Boosey & Hawk: (
www.Booseythai.com), Dimarzio, G&L guitar, MXR.. I think you may find better deal at SG.
Bae-Ngeab-Seng (Cantoneese word): Epiphone, Gibson, Fender, some Music man and some Guild. You may visit its branch called MusicConcept
(
http://www.musicconcept.co.th/index1.htm) at Central World Plaza (former known as WorldTrade Center). Their list price could be discounted for 10% off everything. The price is a bit lower than Sweelee's.
Theera music (roland, burny) and Hongseng (Ibanez): the price is too much marked up.
2) Thai people call it "Hlang-Kra-Tsuang"
Recommended shop is "Chin-Heng-Hlee"
http://www.guitarthai.com/chin/index.html
Other shops there, beware!!! so many fake things.
The used shop, I recommend
www.musiccentercity.com. The price shown in their site you may commit divide by 24.5 to get the price in SG$. No more tax discussion at these price.
www.music-selector.com. Located at 6th floor, 6S-144, Central Rama3 661-4876618 (press 01-4876618 when u arrived Thailand). The stuff there are not that much but the owner named "Mr.Kom" is nice to talk to. You may ask for his advice about your expected NNG Customized guitar.
www.guitarthai.com/musicsociety/musicsociety.html (Music Society). You can reach it by BTS Sky Train. Rad--cha-te-wee station. It's only 1 station next to SiamSquare, which is so near to Central World Plaza.
The recommended path a 1 day stop guitar hunting tour.
Rama9 station (by MRT underground train) reaching Fortune Tower go to #1-Music Center (Basement floor). Then MRT to Sukumvit station and take BTS
to Chidlom station. Ask for Central World Plaza. 3rd floor to
#2-MusicComcept (may hard to find but worth to try). Then walk to Siamsquare, take BTS to next station call "Rad-cha-te-wee" get down by the righ-side and go to
#3-Music Society.
Werng-Nakorn-Ka-Sem is the alternatived place to visit. If you have a time left and do not want to back your hotel. Just call Taxi from the same side of Music Society and head to the place (but not over 4pm regard to traffic). It may be better to visit Werngnakornkasem the next day at morning. You could save your taxi cost. You could visit Music selector after finish wandering around Werngnakornkasem and may ask Mr.Kom for visiting NNG factory because it's about 2-3 km from Central Rama3.