These stones are : dusk stones, sun stones, water stones, fire stones, shiny stones, leaf stones,
Win first prize in the bug-catching contest to win an elemental stone. Also, sometimes people will call you to give you an elemental stone.
If you win the bug catching competition is hg/ss you usually will get a sun stone. Then you can get the rest in pokethalon dome for prices ranging from 2500-3000 points. Once you make it to kanto you can bye them in the apartment store.
Also you can buy them at the Pokeathalon dome if you have 3000 points. Not all stones are there everyday though...