eBay would certainly be your best bet, probably as part of the King’s Quest Collection or the Roberta Williams Anthology
Keep in mind that playing the Windows version of of King’s Quest VI,
which contains much better artwork in dialogue windows, is problematic
in Windows XP. Your choices are to play the DOS version (with lower
quality dialogue box art), to use a version of Windows earler than XP
(possibly by dividing your hard drive into two partitions, one loaded
with XP, the other loaded with an earlier version), or running Windows
3.1 through Dosbox (which, of course, requires getting your hands on a copy of Windows 3.1).
Another possible solution to running the Windows version of the game under XP can be found here, but I found this solution to create too many bugs to play the game properly. It may work better for you, though.