You need a voltage supply ranging 10 - 60 V. Or atleast 10 - 30 V.
Based on the voltage change, the diameter of the obtained nanotube varies. If you have any plan in future to try an application with different nanotube diameter, better to get a wide ranged DC power supply.
The above mentioned range is for pure Ti sheet, since you have mentioned as Ti alloy, the voltage requirement for proper nanotube formation varies with the material composition. Please check to that.