Was in Bangkok recently. No offence to the Thais, if you're coming from Hong Kong, I believe racquet prices in Hong Kong are more competitive compared to prices in Bangkok. But its fun window shopping in Bangkok!
If you're looking for Yonex, there are Yonex showrooms at Seacon Square and Central City Bangna. You can get Yonex racquets, tennis shoes, strings, grips etc.
There's a Wilson store near National Stadium BTS and Tesco Lotus (a hypermart) if you're looking for Asian versions of Ncodes.
Adidas and Nike apparels are available at major department stores - Siam P aragon, Emporium and Central Chidlom. You can also check out other brands like Volkl, ZONS, Pro Kennex, Fischers etc.