I have a nice Oak Pennywhistle D. It HAD the best tone of all the various whistle I had, until the plastic fipple developed a crack that ultimately broke completely.
Is there a source for buying just a fipple, or am I stuck with buying a whole new instrument?
I don’t know of a place where you can buy an Oak mouthpiece.
Now, you could order a Hoover Whitecap for it. 
If you’re trying to minimize your expenditure, it is much cheaper to buy
a new cheapie whistle, than to try to buy a replacement head. That
being said, a Hoover whitecap or blacktop is a pretty cool replacement
if you’re willing to spend $35. It definitely turns the Oak tube into an
instrument worth (at least) $50.
Take the plunge and go for the Hoover White Cap or Black Top. Just about as good as it gets!
Mike
You could buy a not-as-nice-as-an-Oak whistle, and then use the fipple off that. As an aside, I make my own whistle bodies out of type “M” 1/2" copper tubing, and use the fipples off various children’s whistles/slide whistles (you know, the cheapie plastic ones). I know, it sounds cheesy, but it works quite well! 
Whitecap’s and Blacktop’s are good whistles. They have very stable sound and great craftsmanship.