Some weeks ago I ordered a third-party battery grip for the Nikon D800. The original grip from Nikon ranges between 300 and 350 Euros, now I paid 60 Euros plus 11,40 Euros custom for a grip made by MeiKe. A smiliar grip is available from Pixel Vertax (around 90 Euros) and Phottix (also 90 Euros). Both third-party grips are available on Amazon and eBay, just search for, „MeiKe“ or „Pixel Vertax“ in conjunction with „D800“.
The built quality is a little lower than the one of the original grip (I have an original MB-D10 from Nikon for the D300s and compared it to that). By that I do not mean that it feels cheap ! It is just that when you have both grips in your hands you feel a slight difference in favor of the Nikon grip. The buttons on the MeiKe feel a little less smooth when pressing, the dials are a little „rattle-ish“, the material sounds „plasticky“ when tapping it. But let’s be honest. As long as the grip does its job and does not fall apart it is perfectly ok. Remember – the grip does not contribute to the image quality in any way. It is merely an attachment to extent the battery life of your camera and giving you a comfortable grip when shooting in portrait mode.