I would like to learn to play a song that a guitar player has recorded in F#m and am wondering which key whistle I should use or if there are a couple of whistle keys that I could choose from? Any help would be appreciated. Thanks…
You should be able to play it on a A whistle or even a D if you half-hole the G-sharps.
While not a common whistle, you could also play it on an E whistle using “G-fingering”. Dixon and Susato both make E whistles at reasonable prices.
F# is the relative minor to A major, so an A major scale is likely going to have all the notes you’d need. That gives you the choice of an A whistle or an E whistle (using “G scale” fingering). If I’m reasoning this out correctly, you’re likely going to want to go with an E whistle since that whistle gives you xxxxxo as your low F#. In comparison, on an A whistle, the low F# is xooooo, guaranteeing that almost everything you play will be in the whistle’s second octave.
yep, Jim has a great point. although an A whistle would be the relative major to F#m, an E whistle would be a better option.
for example, most of your minor Irish tunes written for D whistle are in Em, not Bm (the relative minor of D). plus, you have the option of the Dorian mode (playing a C# in your Em scale on a D whistle), which is like a “classy” minor. little extra attitude, like a minor scale with sunglasses.
long story short, E whistle.
It was bound to happen eventually …stopped clock …blind squirrel …etc. 
I didn’t know that but it makes perfect sense since you’d likely play anything in Em on a D whistle. Writing the tunes in Em falls right in line with the fact that the vast majority of whistle music is written for playing on a D whistle.
It depends on the actual notes that occur in the tune. Some F#m tunes are hexatonic or pentatonic (so you may not need the G# or C# at all) and others have funny ranges.