Hi Colin
If I am not mistaken, the 637 replaced the 636, but the blades are the same, so anyone selling Kity stuff should be able to supply.
In fact, there is nothing special about the blades, they are plain tool stock (no slots or notches or anything) so any sharpening service shop should be able to supply you with a generic pair. I've just measured my spare set at 260x16.5mm.
If you still have trouble, PM me as I have spare set of Scheppach blades, brand new, which are the same size.
HTH
Steve