I own a boxwood alto recorder by Moec. It gives a very distinctive reedy tone. Moec supply large quantities of boxwood instruments. Most of the main recorder makers offer boxwood. It may be worth contacting these people to find their suppliers. I cant think that they would be reluctant to pass the information on as the quantities used by pipemakers are unlikely to dent their needs. I took my Preshaw pipes to the pub for the first time last night with the folk band( sorry for swearing) and the landlord still bought me a pint. Things are progressing well and people have said what a great sound they make. Thanks Martin.
Tim H
I have found most users of instrument woods willing to part with the species of wood that they use, but are reticent to disclose the source due to the small quantities of instrument grade wood available at a particular supplier. However, as always, I am prepared to be shown wrong by all of the pipemakers who view this forum giving up their sources. 
dave boling