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.
I get disposable knives for my Kity from Tool Mart which I have found to be better quality than the Kity items. The guy who runs it used to be part of Stayers who were the Kity importers. They list 260mm HSS resharpenable blades on their site