If you're not worried about what the whistles look like, don't be afraid to carve the holes up or down. Set the mouthpiece at a true note and slot the other holes to suit. If you go too far just use a bit of tape to flatten the note. One of my generation D whistles has a slotted C# hole which I tape down to give a really nice C natural. It suprised me how far I had to slot the hole though.
As on another thread I will give this advice-Always have the means on hand to alter pitch by mouthpiece positioning, and correct individual notes with file or tape. Sometimes you have to do it almost on the run.
I do not have any expensive whistles, and I must say I would be reluctant to hack at anything too expensive or rare.