IDE drives are a dime a dozen these days. If you want a plain run of the mill CD drive, you can get one anywhere.
If you're getting some kind of writing device (DVD +/- RW, CD-RW/DCD) one thing I suggest though is make sure you can get firmware updates for it.
As for places, check out "mom and pop" (local) shops. Computer repair places as well.
To celebrate the passing of the Troll Amnesty (and for otherwise no reason at all), I present to you the very best of Trolling here on The W. This Troll Moment of the Week is brought to you by one of the greatest(?) Trolls to come The W;
"TopTenPro" , who brought us the following such pieces of wisdom as: br>TopTenPro: I know this is going to be banished, but there is a point I want to make. Why is there not a woman that wants to be loved as much as I want to love?"
CRZ: "Here now are the Top Ten cruel responses I could whip up to this post:" (followed by a rather funny list).
Off topic I know, but I know that sites like xbox-scene.com were flooded when the system came out. Recently made systems are being made more stable it seems, but it's still a major issue to anyone that bought one around launch.