Odd, there's definitely one in the 2005 650 toolkit
I'll measure mine tomorrow (the C-spanner that is), assuming it's the same as the thou.
Busters Accessories do a range, think they do an adjustable one too, but their mini one that fits onto a 3/8 ratchet fits my 650 just fine.
EDIT: I think the one from Busters was a fiver btw