If you have a 9 speed XT drivetrain, your dealing with model numbers of shifters, derailleur's m750, m760 or some m770's, when they went to 10 speed with model m773 and above. If your shifter is a clamp-band shifter (mounts to handle bar via its own circular clamp), you can use any mechanical or hydraulic brake lever which will also clamp onto the bar independently of the clamp band shifter. If your XT shifter is an i-spec mount (integrated with your current XT V-brake lever) then you're dealing with the i-spec B system, which is NOT compatible with i-spec II.
However, there are some work-arounds and options available. If you are satisfied with mechanical disc brakes which are activated by a cable system, your current v-brake lever will be appropriate and you don't have to change anything in your cockpit. An m750, 60, 70 model brake lever is i-spec B type as are the same series of 9 speed shifters.
If your heart is set on hydraulic brakes, you'll need i-spec B compatible brake levers (the m8000 XT is I spec II and not compatible with i-spec B shifters). Some hydraulic brake lever model numbers that are I spec B: BL-675 B (Deore LX), BL-m820 (Saint), BL-S700 & 7000 series (Alfine), BL-785 B (XT). There are more possibilities if the m8000 series lever needs to be used. Read on.
Another option holds if you have the i-spec B shifters currently and want to run the m8000 levers. This would entail buying the compatible top part of the shifter that includes a bar clamp. Essentially you're replacing the i-spec part of the shifter with the clamp. This will also come with the indicator window. I do not believe there is an optional part that is the clamp without the indicator. A clamp on shifter, again, allows the use of any brake lever.
Finally you can mate the incompatible i-spec B shifter to an i-spec II brake lever if you'd like to purchase problem solvers' integration piece. The "ReMatch 1.1" will mate an I spec II brake lever to an I spec A or B shifter.
So you have some options. Power to the people!