Meike MK-D600 is a cheap alternative battery grip for Nikon D600 - Nikon Rumors
Amazon.com: VELLO BG-N10 Battery Grip for Nikon D600 : Electronics
Nikon MB-D14 Portrait/battery Grip +EN-EL15c Battery Bundle (for Nikon D610/ D600) | London Camera Exchange
Amazon.com : Xit XTNG600 Digital Camera Battery Power Grip for Nikon D600/D610 DSLR Cameras (MB-D14), Black : Digital Camera Battery Grips : Electronics