A comment on another forum (GR Research @ AudioCircle) mentioned that Erse Audio Inc outside Cleveland has closed down. I have sourced both air-core and laminate-core inductors from them for many years, both direct order and through Madisound and other vendors. Almost unique as a source for large AWG wired air-core coils. Digging a bit more it appears that owner/founder Jim Smolik passed away 2 years back. Sad for the family and sad for the many of us in the DIY space.