I do not believe that it is being sold that way. Since I am not absolutely sure, I have pinged one of the marketing team members to get a definitive answer. I will let you know when they respond (if they don't respond directly to you)...
The motherboard for the 6 series NUCs are not for sale separately. However, the motherboard for this unit: Intel® NUC Kit NUC5i5MYHE and Board NUC5i5MYBE Mini PC (and the i3 version) can be purchased separately.