Where can you get dragon balls from Dragon Ball Z

If you want to buy it, go on Ebay and search Dragon Ball (z) [Kai].

If you're talking about the show, the dragon balls may be found on any planet on which a Namekian sorcerer (has magical powers, like healing, but the most important one being the ability to create dragon balls) lives. In the show the only planets we know that have dragon balls are, Earth (sorcerer being Kame) and planet Namek (sorcerer being The Grand Elder, his name is unknown to all but his father).