That hasn’t been available for a fairly long time now. Actually, about the time MSD bought out Mallory and killed off most everything.
I have the advance key and a few springs left, but I hoard the springs.
If I had a full set of them, I send them to a spring manufacturer and have them wound.
I have never been able to find out who Mallory had make their springs. If you can find that out, you may be able to buy some springs.