As a matter of interest, Tony Nailor of Spectrum Communications can supply both iron dust and ferrite toroids. He also supplies binocular ferrite cores as used in 'Wellgood' clone active antennas, and trimmer caps as needed when padders are used with home-brew coils an an alternative to cored coils, as referred to in Joe's posts above.
http://www.spectrumcomms.co.uk/Components.htm
Forum members may recall the enigmatic 'RF Haigh' who - for some years - was a prolific writer who wrote many articles on beautifully constructed receivers and other projects (Such as the Radio Bygones Wobbulator project). All of his receivers had hand-wound coils, and in one of the magazines he did publish details of how to go about it, but I can't recall when and which publication.
Internet is of course awash with information and publications which can be downloaded such as the one at this link.
However, for the likes of me, within a couple of pages I disappear down a rabbit hole:
https://ia904601.us.archive.org/10/i...ansformers.pdf
Looking back it's remarkable that small firms such as Osmor, Weyrad, Denco and REP were able to do all the calculations, and to manufacture such quite intricate coils. Whether or note the fine wire was actually Litz, as we assume, rather than cotton or silk covered, remains to be seen.