I want to grow a cactus/cacti!?

ok so i want to grow a cacti, my mom bought me a fake one and it just sit there...never dying or growing....BORING!! so i want to grow a real one from a seed, i know all the requirements to grow one its just the seed problem. i want to grow a cacti that is round like a ball. don't want the cactus that are like twiggy or branches flying everywhere. just want a normal round cacti, but i don't know the name, so can you guys please tell me the name of it? and leave a pic of it is also going to be amazingly helpful! thanks :P