I’ve joined a concert band and it’s great fun but all the music is in Bflat ,Eflat, and A flat. I can’t find a suitable whistle. I already have to transpost to d fingering and even the recorders are str 
uggling.
If you mean that the key signature has these three flats in it, the music is in the key of Eflat. I think an Eflat or a Bflat whistle would work for this, but get a few more opinions than mine first! I believe a Bflat whistle would allow you to play below the concert Eflat (fingered G on the Bflat whistle; so a piece played in the key of Eflat would be played as if it were in the key of G) if that was required in the piece, so that might be a good choice. Of course, I don’t even know if there is such a thing as a Bflat whistle!
Robin
Of course, I don’t even know if there is such a thing as a Bflat whistle!
Almost everybody makes a Bb.
If you mean you find the tones of Eb, Ab and Bb in the same score, the major key is Eb.
A whistle in Eb (available in high as well as low from many manufacturers) will play the keys of Eb and Ab easily. Bb should work too if the whistle reaches easily the top of 2nd octave and if you can half-hole a G# on a D whsitle, or if the whistle accepts a cross-fingering like by xxo xxx.
However for the Bb key, an F whistle may be more advisable than both Eb or Bb: if the music your band plays is mostly Scottish, then you’ll need often that Ab below the Bb, which a standard Bb whistle won’t give you.
Or is it a brass-band?
Generation make Eb, Bb and high F whistles the Eb can be particularly good and would be a good start.
Eb whistle second major key (G fingering on a D whistle) is Ab
Bb whistle second major key is Eb.
F whistle second major key is Bb.
Which whistle you choose will depend on the tune and how it fits the basic 2 octive range of the whistle.
See http://www.chiffandfipple.com/whistlekeys.html for full details
Joe
I will try to get a Generation Eb as I do like the Bflat I have. Needs to be loud as well to compete with the trumpets and cornets. 
Wouldn’t say it’s that loud, but quite a nice sound…or had until my 21 month daughter decided it was HERS and hers alone…and yes she does sit down while playing at it…anyway I’ll get another one one day with less chew marks on and less peanut butter stuck inside the tube too no doubt.
Trisha
If you haven’t already, check out the nifty chart Dale put in the “Deciphering Whistle Keys” section. edit- or basicly follow Joe’s link.
Jack ‘Nifty Fingers’ Orion
If you are a low whistler, you can get Bb with the low F, as well as Eb and Ab with the Eb. In addition, the tuning slides of tunable Dixons can be pulled out far enough to go a half-step (one flat) down with a little creative breath pressure.
I have a gen Eb that I use for playing along with highland pipes. It does cover the keys you mentioned quite well. It’s fairly in tune without itself, though I had to pull the head flat and back fill with wax under the windway to be able to hit second octave Eb.
TW