5.25" slot-loading DVD drives are almost non-existant now so I reckon your only option is going to be to get a slot-loading notebook drive (like the OptiArc AD-7630A) and then an adapter so you can ...
I got a Pioneer 106S DVD-ROM about 6 years ago and it's been serving pretty well. I think the ease of slot-loading is awesome, however recently these kind of drives have been disappearing off the face ...