Last time I was in Japan, they still made blue blades that compared in hardness to the old Gillette one. I bought a stack! They do show up on e-Bay occasionally. I still have my "Little Gem" plane from college (45th reunion coming up!) If you have contacts in the far East, have them check on availability.
It seems to me that some of the free-flight guys also sell blue blades.
One interesting source for a pile of the blades once was an old house that was being renovated. There used to be a slot in the back of some medicine cabinets for old blade disposal. One of these happened to be removed, and there was a pile of Gillettes neatly preserved in gooey soap. Wow!
I also have a German razor plane, and the blades can be damaged, but I haven't worn one out! I checked, and Hobby Lobby still has German planes and blades, but the blades are $2 each. But then you get 2 edges, and can prolong the life by occasionally flipping the blade over to reverse the wear pattern. Probably can sharpen them with the "Scary Sharp" technique, too. (Do a Google search on the term, it is cheap and easy to do incredible sharpening this way).