Building a computer

hey guys, i guess this isn’t other software more other hardware but i am building this for use with, mainly, Blender and Unity. im planning on building a computer that i want to have simmaler specs to the 12 mac pro (this years), but with some alterations (mainly a nVidia GTX 690 or Titian). So i was wondering where i should buy the parts as i cant find any suitable motherboards on newegg or NCIX.