Do not buy an extended slave rod. I had the same problem and it was the clutch fork/pivot ball. If you have bled your hydraulic system properly, gotten a new master/slave, then you really have no alternative but to drop the transmission and take a look.
Also another easy to check if it's the tranmission is to look at the fork tongue that sticks through the window on the trans (where the slave rod pushes against). It should be in the middle, if it is skewed (left or right) then you know it's your mechanical system.