Bob Ross® Phthalo Blue Oil Paint
Bob Ross® Phthalo Blue oil paint is used very often in the wet-on-wet method. Usually, you'll use Phthalo Blue for sky scenes, water, and often as part of the shadow color on snow-capped mountains. If you paint often, the 5 oz Phthalo Blue tube will be the way to go, otherwise, the 1.25 oz Phthalo Blue tube should last you quite a while.
If you cannot find a local retail outlet for Bob Ross® Phthalo Blue then you might consider buying from a reputable, high-volume art supply dealer online. The normal list price for the 5 OZ tube of Phthalo Blue is $17.49 and the 1.25 OZ is listed for $7.29 but you can buy it online for far less. The buy link below doesn't commit you to buying but will take you to the merchant website where you can see what is available.