It’s going to be next to impossible to find a tunable D by itself since Steve Harper only made the tunable D as part of a D/C set. So you need to be looking for the set, and again, not easy to find.
I have nontunable D and C Harpers and the tuning is the most dead-on of any nontunables I’ve owned, so I wouldn’t be too concerned about finding a tunable D. I use the D in sessions, in the studio, and with the band with absolutely no tuning issues.
Last time I called the Hobgoblin U.S. store (a couple weeks ago) they had one Harper Classic D left in stock, so give them a call. They don’t list the Harpers on the web site. 1-877-866-3936
Yeah, Harpers are great to play…I just got mine from Hobgoblin US a few weeks ago, and I can’t see to put it down. Dead on in tune, and easier for me because of the larger holes on it (at least compared to some of my other whistles!)