Take a pic of it. It'll be either the standard slim bay or if it's hotswap-able, it'll have Dell's hotswap kit wrapped around a standard slim drive, which you can take off and put on the new drive .
Either way, any slim 12.7mm optical drive will fit. It comes in either 50pin IDE or mini-SATA. Find out what your current one uses. The hardest part is getting the existing faceplate to fit on the new drive (hence I need to see a pic of your current one).
They're ~$56-60 on fleaBay for a BD-ROM/DVDRW combo drive. I use one in my HTPC.