Phil Bleazey makes some very nice wooden whistles with tuning slides, and not expensive for an instrument of that quality. I've got one of his flutes, and can vouch for his craftsmanship.

His wooden low D whistles, while more expensive than many ally tube ones, have a beautiful sound - the bottom D has a lovely buzz.

He might be difficult to get hold of at the moment though- his wife Cathie has recently died after a long illness.