Get Pentax 645z at B&H Photo.
See Pentax 645z: Hitting the Bullseye on Which Target? and Pentax 645Z 51-Megapixel Medium Format DSLR, 13 Pentax Lenses Now Avail in USA and Pentax 645Z: Does Not Have EFC Shutter (Electronic First Curtain).
Also my review of the Pentax 645D and lenses.
It’s a shame that the Nikon D810 is a month or so off, because I’d like to compare it (using the Zeiss Otus 55/1.4 APO-Distagon!) with the 645Z. The 645Z sensor is notably larger, but 50 megapixels is only marginally more than 36 in linear resolving power terms.