There are a no. of guitar stores in Bangkok, just gotta know where to look.
Siam Paragon (next to Siam BTS): 2 guitar stores there, mostly selling PRS's, acoustics and Washburns.
Siam Discovery Centre (opposite Siam Paragon): One guitar store on the 3rd floor.
Central Lat Phrao (Pahon Yothin MRT): One small store with Parker Fly's, BC Rich, Washburn's and some other stuff.
***Fortune Town (next to Phram 9 MRT): There's a shop in the basement, mostly selling second hand instruments, but they do have brand new stuff.
***"Wurng" (Chinatown area, near Hua Lamphong MRT): Cluster of guitar shops in that area, but i think most of them are closed on Sundays.
Here's the address to another store:
Rinton Music Products
F and V Co., Ltd.
46/ 27 Sukhumvit 105, Bangna, Bangkok 10260, Thailand
Tel : 661859-3232, 669-771-4744, 662-748-6219
Fax : 662-745-8029