Citing a link that was removed?
If the page was up for a while, you can probably obtain a copy via the Internet Archive.  Once you've got that cached version, you can host it yourself, with appropriate attribution (i.e., the fact that the original source is missing, its original location and authorship, time that the snapshot was taken by the Internet Archive, etc.)

This could be a problem from a copyright perspective, but if the original material truly is abandoned, then you've got nothing to worry about.  If the owner does contact you, then either they'll likely be willing to post a copy (or its equivalent) again, obviating your need to host your own copy.  Only in the rather unlikely case they both want the material taken down and are unwilling to repost it themselves will you lose the source.