Unfortunately, there is no easy way to get an oval stone in Black 2, other than those dust clouds. The stone can only be bought in White Forest, which is exclusive to White and White 2. The dust clouds are the only other way to get the oval stone, and they appear in Chargestone Cave, Mistralton Cave, Clay Tunnel, Twist Mountain, Underground Ruins, Victory Road, Wellspring Cave, Seaside Cave, and Giant Chasm. For every Dust Cloud you encounter, the chance of finding an Excadrill or Drilbur is 40%, while the chance of getting a gem is 50%, and the chance of getting a stone is 10%. Of those stones, there are 12, and supposing that each stone has an equal chance of appearing, that gives you a 1/12, or approximately 8.3% chance every time you get the 10% chance of finding a stone, meaning that all in all, you have a 0.83% chance of finding an oval stone every Dust Cloud you step on. This basically means it's all up to RNG, so the best way to get the oval stone is to keep trying. Hope I helped!