Where can you get a star ultastar 9mm magazine?

Which caliber? The Ultrastar was made in 9x19 Parabellum, 9x21, and .40 Smith & Wesson. Triple K products manufactures them for the 9x19 and .40 S&W, at a cost of $42/per. If you have the 9x21 model, you'd probably have to go to a European distro, such as Smart Reloader. Of course, there are always your sites such as Gun Broker, Auction Arms, or Guns America.

The OEM - Star Bonifacio Echeverria, S.A. - went defunct in 1997. The remnants of Star, Astra, and Ipar Guns merged to form a company called ASTAR, whose products have not been made available in the United States.