Adapting a Pentax lens to my Nikon?

What adapter should i get to put my Sigma DC 18-250mm (From on Pentax K10D) onto my Nikon D7000? Will there be quality loss?
I'm having troubles

Actually it already has quality loss by being a Sigma 18-250mm lens. Anyway, the Nikon mount is narrower than most, so I doubt that there's a compatible adapter between Nikon F body and Pentax K lens mount.

Http://www.ebay.com/sch/i.html?_nkw=nikon+lens+to+pentax+pk+adapter

It seems there's an optical element (glass) in the adapter to correct focusing to infinity. That will severely degrade optical quality. Then again, the adapters I found doesn't seem to preserve automatic functions so be prepared to shoot manually without the benefit of a lightmeter. Your biggest problem would be aperture control. I see no way of controlling it so you would be stuck at minimum. Very dark.

Much better to get a true Nikon fit lens. Such adaptations are never satisfactory, and probably won't have all the auto functions that a Nikon-fit lens would have. If the "adaptor" includes any optical elements (bits of glass) then there will certainly be loss of visual quality. Somebody should have told you to upgrade your Pentax body (rather than move to Nikon) if you wanted to retain the use of that lens.