A whistle in D will allw you to play along with MOST of the Folk World, except for the awkward beegars who us things such as B/C melodeons.
As for WHICH "brand" of whistle, a lot depends on personal taste. There are the ones with the plastic end and "silver" or "brass" barrel include names such as Generation and Feadog. There are the ones with a tapered barel and a wooden wedge at the blowing end - Clark whistles are in key of C but Shaw? are in a variety of keys.
Some people like the "breathy" tone which the Clarke whistle produces. I personally prefer the crisper tone of the plastic mouthpiece. There are people who will fight the case for each type.
Quack!
GtD.