Hi David,
first of all - I have no idea where to get one of these; if you should find out, I'd be highly interested myself.
"Duduk" is the bulgarian name for a whistle, and "Vlashki" means "Wallachian" (used synonymously for "Romanian"). "Vlashki" instruments and music can be found where the two countries border, along the Danube river.
At first I thought this instrument was a Romanian caval (not to be confused with the Bulgarian kaval) but a closer look showed it seems to be a very big six-hole whistle (in Romania it would be called a "fluier mare", but it is not very common there). Fipple, windway and labium are at the back of the instrument.
The sound you're after is called "kaba" in Bulgarian kaval playing (which also uses a weird embouchure similar to that of the ney). A very similar sound is achieved by romanian caval players by half-covering the window with the lower lip (you can hear it here:
http://www.youtube.com/watch?v=_GZdynReldQ). Basically, the same technique can be applied to any duct flute with the windway on its back - which may well be the main reason why most East-European duct flutes are made this way.
This instrument maker makes duduks as well as bulgarian kavals (which he also offers with a duduk mouthpiece):
http://www.flutemaster.net/ - I guess he would be able to make a low-pitched duduk, if you asked him. I know nothing about the quality of his instruments, though.