I got mine directly from Cybex, and you're right, it's really hard to find otherwise.
So, the top shoulder height is 21" MINUS TWO FINGERS ... really. The measurement to the bottom of the headrest wings and belt guide is right at 21" BUT there's a note in the owner's manual that says:
"ONLY use this booster seat with the bottom of the Head Protection positioned at two fingers width above child's shoulders."
I personally find imprecise measurements like that really annoying -- whose fingers? Andre the Giant's, or my 18-month old's? A 5th percentile female's, or a 99th percentile male's?
Anyway, the bigger thing I'm disappointed with at this point is that it's not going to work in our '11 Honda Pilot - the belt stalks are too long and the latch plate interferes with the belt path.
It'll have to go in our secondary vehicle, an '07 Sierra which has flush/rigid belt stalks. It would also probably work in the 3rd row of the Pilot, but I don't want her there, so for now I'm putting her back in Olli in 2nd row.
Once I get a bit more caught up around the house I'll try to take some pictures and compare with the Parkway ...