In the UK the Early Music shop sells recorder roll bags. They come in 3, 6 and 9 slot versions and work just as good for whistles. You won't get a one or two piece low D in them, but my A and D Whistles go in fine with the heads off and the Gen Bb just fits. Go to http://www.earlymusicshop.com/ and search for "roll bag"

My daughter knitted me a bag for a descant recorder. She made it a little large so it actually holds two recorders and three whistles :-)