Hello there:
in your request you gave no information about what you want.
"A ferrite" can be a low-frequency ferrite core used in power-supply transformers, a ferrite rod antenna used for medium waves, a microwave ferrite suitable for a microwave circulator, or a ferrite core in a VHF coil.
Like with any other component, you should be able to SPECIFY basic parameters needed.
Try then to google phase shifter manufacturers. The problem can be that generally, bare ferrite material is rarely offered. Device makers prefer to make their ferrite blocks and install in finished devices.