The notched type looks like a pretty standard C spanner, the Thorn version isn't bad value, but you might find one locally in any decent tool shop, or if you're sure of the size a bargain one on Ebay.
I'm glad they didn't stick with that design, by the time I bought a MkII it'd changed to the two hole design. A pin spanner is the correct tool for that, but it's as easily done without by sticking a couple of screws in and using any thin spanner.