We used many ferrite cores in SMPS and some HV designs since 1970s and have a shelf of old stock, as well as the original printed catalogues. We used Ferroxcube, Stackpole, TDK, in EI, EE, ER, and other shapes, most have no gap, in usual power matériels for 20-50 kHz.
Most standard cores sizes are available today from Epcos, or TDK, in EU. US sources are Fair Rite, TDK.
If you can post clear photos and dimensions I can look at what I have, we might get lucky.
Happy to assist,