This is actually not out of place, since these forums now cover the entire X-Wing series (including TIE Fighter).
While looking for a site with TIE Fighter mission hints to help someone on the TFN boards, I found Allen's TIE Fighter Page
and hit the jackpot, as he has not only 5 custom TIE missions that he did himself (and can be downloaded together), but also a bunch of missions done by others (scroll down past the links, most of which no longer lead anywhere, for these).
Note: These missions were originally created for the MS-DOS version of TIE Fighter, and should work in DOS versions by unzipping the archive and running the batch file (*.bat) included in the archive. They should also work in TIE95 (Windows version), but you should NOT
run the batch file; instead, open the batch file in a text editor to see which mission the custom mission replaces, and then replace the mission manually (be sure to back up the stock mission so you can restore it when you're done playing the custom mission).
: This may not be the end of it...stay tuned!