Bob Ross® Prussian Blue Oil Paint
Bob Ross® Prussian Blue oil paint is another heavily used color with the wet-on-wet method. Usually, you'll use Prussian blue for sky scenes, water, and often as part of the dark base mixture for mountains. If you paint often, the 5 oz tube will be the way to go, otherwise the 1.25 oz tube should last you quite a while.
If you cannot find a local retail outlet for Bob Ross® Prussian 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 Prussian 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.