Bob Ross® Sap Green Oil Paint
Bob Ross® Sap Green oil paint wasn't used by Bob in his early PBS TV programs. It was added to his oil colors later and it is an essential component now in mixing all the beautiful green hues used in landscape paintings. If you mix Sap Green and Alizarin Crimson together you'll get a beautiful brown color. 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® Sap Green then you might consider buying from a reputable, high-volume art supply dealer online. The normal list price for the 5 OZ tube of Sap Green 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.