Ken, I don't think a DB part exists. I went to Petrucci, which provides free downloads, but they don't have parts for this, so I can't help you. (I just wanted to remind you of what a great guy I am.)
Petrucci is a great resource, though; they have tons of free stuff. Go to
http://imslp.org/wiki/Main_Page
Click on 'Composers', click on the composer you want, and you'll get a list of available works.
The full score for the Holst shows a staff for "Basses", but just as you say, that's really tuba writing. One option is to click on 'Full Scores' and select the Condensed Score. It's like a piano reduction, but you'll see notes on the bass clef for which instruments are playing. It will be clean, and you can transpose up an octave and write in where necessary. That's the best I can do.
I'm doing some concert band stuff now, and I do this all the time. I simply can't sight transpose from below low C.
Good luck, you old goat.