anyone know of keyboards with the same sized keys as the sk's?
They aren't individual keys, its more like a plastic sprue of keys, sometimes called a keyboard tree. If you need to replace one key you have to replace them all.
I'm sure there was some discussion on this before. Do a search for keybaord trees or similar.
Some others in the SK series have the same size keyboard trees.
Not sure of your situation, but if you broke off a key and still have it you can take the SK-1 apart and glue it back on with some strong superglue and it should still work just fine. I did that with an SK-1 and a PT-100 and the keys have been just fine ever since.