Ly Nam De is the place to start, but I don't recall ever seeing a mac shop there either. Someone down there should be able to direct you to a mac supplier though. If you come up dry there, there is a big shop near the WWF office, along Tran Phu I believe that might be able to help/direct you. Sorry I don't have the address, but I used to see it while riding the #50 bus going out to Kim Ma.
The place!!! Make sure you walk the whole length - there is a break in the middle where there are no shops, but then they start up again.
However, if you are in Bangkok some time, there is a shopping mall there that beats Ly Nam De hands down! I will see if I can dig up the name and post it.