Actually Maplin sells PP9 at about that price so not too horrific.
Also try Amazon.com and google with US part numbers!
But cheaper to make your own with 6 x Alkaline AAA (or 5 x AA will fit if soldered and whatever it is will work 5V to 7.6V). Not sure if a 2 x 3 layout 6 x AAA holder fits 34 x 36 mm x 70mm, but seems plausible. One I've not "done" yet.