If it's a PC (as apposed to a laptop), then you can pritty much buy them from anywhere. Remove the drive first - open up your PC. There is normally 4 screws holding it in place (sometimes holders depending on the PC), and 2 plugs. 1 is a 4 pin power connector. The other will either be IDE (ATA Ribbon cable) or SATA (red cable).
For the purposes of making the swap as simple as possible, try to make sure that your replacement drive matches the connections you have on your current drive (IDE/SATA).
You can buy them online (for example e-bay, just look for internal DVD Writer /
RAM). However you can also buy them from your local computer shop, or from Comet, Currys, Dixons, anywhere like that for around £30-£40.
EDIT: Marked as Answered.