Adding New Hard Drive...

I want to add a new hard drive to my Dimension 3000 (which only have one drive), but it appears that it has only one drive casing for one hard drive to be installed into it...but it does have this empty space next to it, and I would need to acquire an additional drive casing for placing a new hard drive to it-- where can I purchase one of these harddrive casing to be installed into my Dimension 3000 casing? Thanks in advance to anyone that'll respond to my inquiry.