Amazon.com : Phottix Battery Grip BG-D3100 for Nikon D3100 and D3200 : Digital Camera Battery Grips : Electronics
Nikon D3100 Video 2: Operation, Battery, Memory Card, Lenses, Flash, and Functions Manual - YouTube
Amazon.com : Neewer Professional Vertical Battery Grip Replacement for Nikon D3100/D3200/D3300/D5300 SLR Digital Camera, Works with 1 or 2 Pieces EN-EL14 Batteries : Electronics
Nikon D3100 DSLR Camera Highlights & Overview - YouTube