Indeed I have.
Lundahl buys a few different types of amorphous magnetic material for their transformers. They use a high permeability cobalt-based material for the small signal trnasformers and a "normal" mu iron material for the large signal tube transformers. I am not involved in the manufacturing, so I have no idea where they get them, but I am told by others that some of the US manufacturers have standard lams available in both nanocrystalline and amorphous materials. I don't know who they are, though.