The minor plugins are not available for Xbox and have usually not been included with any PC release of the game, including the Game of the Year Edition and the Steam digital release. The major expansions Tribunal and Bloodmoon can be obtained separately on the PC, and are also bundled with the Game of the Year Edition on PC and Xbox.
None of the minor plugins can be obtained from the official website anymore, but they can still be downloaded directly from the links on this page. Helm of Tohan was originally released online as a free pre-order exclusive, while the rest were published by Bethesda on their website. This is a list of official plugins released by Bethesda exclusively for the PC version of Morrowind.