You can get the fluid right here on fast lane, they sell the filter also. Having said that, where did the oil go? As the oil displaces the nitrogen in the sphere the ride usualy goes from bouncy to stiff since the oil cant be compressed. There is usually a lot of gunk in the reservoir when accumulators fail. When the accumulators are changed (cramped&messy) the lines are usually flushed, the system usually self purges, have someone look at it. If the accumulators were never changed, and the tank topped up maybe that is where the oil is.