When Piccolo/Kami came to earth after the terrible plague on namek he created the black star dragonballs( which you will only see and hear on dragonballgt but after suffering the witness of world...
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,...
There are 7 Dragon balls. However according to Master Roshi's story the Dragon balls used to be one big ball how ever it generated to much power so it split into 7 balls.