There is no one Mystara campaign setting product, just the gazetteers and a myriad assortment of accessories and boxed sets that TSR put out years ago. Most of the Mystara products are for the D&D setting (basic, expert, companions, masters, and immortals boxed sets), though there were several 2nd edition AD&D products put out for the setting.
A nearly complete list of Mystara products can be found
here on the Vaults of Pandius. There are also several articles that have appeared in Dragon magazine recently that are either explicitly set in Mystara or Mystara related- a couple of articles in #315 explicitly for Mystara, as well as articles on two Mystara races, the Lupin and the Diabolus, in Dragons #325 and #327. There is also supposed to be something in the upcoming "Campaign Classics" issue that is due out in January.
The setting itself is still very much alive and well, if only in the hands of its fans, who maintain an active mailing list and "official" webpage at
the Vaults of Pandius. The
Mystara Message Board at the Wizards of the Coast site is also very active, and recently passed up the Ravenloft board to take the 5th most active board on the Other Worlds page! (Yay, us! Watch your back, Planescape!)
There are several threads on that board pertaining to 3.5 edition updates of the Mystara campaign setting, as well, so come check it out!