Can a ceiling light shade be replaced? Where can I find a replacement?
For the kind of light fixtures that you typically get in apartments (i.e. not terribly nice), you could look in the big-box stores.  If you're in the US, I know Lowe's has a selection of them (though admittedly not a huge one).

If you can't find one there, look for a specialty lighting store, where you should be able to find a much wider selection.