Computer Hardware
If you're looking to buy new (or used) computer hardware, the easiest option is Fortune Town
Located here on Google Maps: https://maps.app.goo.gl/umQfxzuhcYkaES6w5
Take the MRT (Blue line) from QSNCC for 3 stops going north and exit at station Phra Ram 9
Use Exit 1 to be close to the entrance of Fortune Town Mall
Go to the 3rd and 4th floors to find a range of computer equipment and services
If you're looking for something cheaper and feeling more adventurous:
JM505 Computers Service at https://maps.app.goo.gl/UvR2BQQmKMKaRC1E9
BSL Computer at https://maps.app.goo.gl/PpAg8nfUGBTGhwwh7
NB: A number of these vendors buy end-of-lease computers and may often have a number of the same model at quite reasonable prices.
Many large malls have an "Apple" shop (some are istudio by copperwired); you can also find Apple devices in Central, Banana IT, and several other larger retailers
There are official Apple Stores at Central World and Icon Siam
Many larger mall have a "phone" section with Telcos, large chains and independent stalls
The independent stalls are better known for phone cases, screen protectors and unofficial repairs
The best known location is 4th floor of MBK CENTER which may be a better option, especially for advanced phone repairs
Directions: Take the BTS Skytrain (Green line) to National Stadium (W1) and use Exit 3 or 4. There is a connecting walkway to MBK Center and you will need to go up 2 floors (it can be a bit of maze to navigate)
Lemon Telecom is located on the LG floor of QSNCC, behind the KFC