After my first few weeks with my little sweetone in D (I’m being good and practising diligently every day!), I am starting to feel a little confident. I can’t imagine getting a whistle in a different key…isn’t that just like starting all over again, doesn’t it just mess you all up, isn’t that just like learning a new instrument all over again?? 
Nope! Most peole (actually all that I know or have heard of) just use the same fingerings on all whistles. Play a tune on a Bb whistle just like you would on a “normal” D whistle and it comes out in the key of Bb. (that’s a little simplified, but more/less…).
Unlike those crazy recorder players who do actually learn a different fingering system for different keys of recorders, or so I hear.
Get another key, and pretend it’s a D. That’s called transposing, you’ll be fine.
I recommend choosing A key whistle.
You can pretend it’s a D whistle.
You can also play D/G/A tunes.With that this gives you a good learning period for music theory and good breathing technique.
Convenient and fruitful choice. 
OK, I think I get it! Seems obvious now that I think about it! Thanx for the pearls of wisdom!