You can find converters on ebay, if you search for the mounts to be converted.
As Bjorn said, infinity focus is a question of mount distances, which then also have to consider the adapter. In a quick look I don't think any of the Leica/Pentax/Contax G lenses will have infinity focus on the Nikon, unless you use adapters with corrective optics which will alter the characteristics. Leica and Contax G have short mount distances so there's no chance there. Pentax assuming they're using the same mount as today, is close but still shorter than Nikons, even before you add an adapter.
Better bet may be to get another body. The Sony NEX range is possibly one of the best to use with older lenses as it has one of the shorter mount distances, an APS-C size sensor, and a wide range of bodies covering most price points. Micro FourThirds cameras from Olympus/Panasonic could be a second choice, but they have a smaller sensor size.
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.