Richard Clark wrote:
. . .

Who makes them capable of 450 MHz or so?


Amidon, but this is not proprietary (although the type specification
may be). There are other manufacturers like Fairite.


I don't believe that Amidon manufactures cores (except, apparently,
custom ones through one if its subsidiaries), but only resells them. The
last I checked, most of their ferrite cores are from Fair-Rite. The
common numerical ferrite type designations of 43, 72, and so forth are
Fair-Rite's.

They also sell powdered iron cores from Micrometals.

Roy Lewallen