Anytime case questions come up I give my standard link. http://www.genesysdtp.com/
great prices, fast shipping. I did exactly as you did, OP. Unfortuantely I ran into the same issue that you did. 7 discs in a standard width dvd case is a tough one. I have not found such a creature. I have both titles you specifically mentioned in your post. For x-files, I only had the original dvd release of season 1 which had 7 discs, then the rereleases for the rest of the series that came in single width cases and only had 6 discs. I ended up using a wider 7 disc case for season 1. For DS9, i used the "Ships of the Line" covers from http://www.rixgrafix.com/
which also are designed to put every star trek series season in a 1 inch wide case.