Sim Lim Square. Just beware of scams and inferior stuff. Some pricelists for you to look at before going there to shop for one.
http://sls.vr-zone.com/Or you can try those Challenger stores if one is near you and you don't mind paying a few more bucks.
Just a note that if you don't feel comfortable opening up your computer casing in order to install the optical drive, or wouldn't want to spend too much time on this, it may be easier for you to instead purchase an external optical drive (which is actually just the same drive plus one case) and plug it into your computer through a USB connection or the similar. Of course, this option usually costs around 20 - 30 bucks more due to the additional external casing.