Yeah, usurp, you gotta go to NewEgg. They're a generic computer parts store, so while they do sell PC-specific components (motherboards, processors, etc), they also sell optical drives, hard drives, RAM, monitors, all that good stuff that works in both PCs and Macs. By the way, you can go with the DVD burner if you want, but they also have a number of Combo drives there. The main issue is finding a Combo drive that is fully compatible with OS X - most of them are not. So unless you're getting the known-good DVR-107D, you should probably go to www.xlr8yourmac.com
and click on the "Drive Compatibility Database" link. It has a huge listing of optical drives and reports on whether they work in OS X natively, with a hack, or not at all.
PS. I just checked the database and it seems as though there aren't a whole lot of natively compatible drives. However, if you download the "Patchburn II" utility for Panther, you can get support for many drives. One that seems to work particularly well with Patchburn II is the LG GCC-4521B. The earlier version of that drive, the LG GCC-4480B, is natively supported without any need for additional software. The fully supported ones are $40 on eBay - do a search for "GCC-4480B" and make sure to check the box that says to search titles and descriptions.