Bob Ross® Van Dyke Brown Oil Paint
Bob Ross® Van Dyke Brown oil paint is heavily used in mixing the dark base mixture for mountains, rocks, and trees, and for quickly laying in large dark foreground areas. You'll use quite a lot of the Van Dyke Brown. 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® Van Dyke Brown then you might consider buying from a reputable, high-volume art supply dealer online. The normal list price for the 5 OZ tube of Van Dyke Brown 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.