Seems that you can at least buy a Quantum SA.45s Chip Scale Atomic Clock for a mere $928:

- https://www.microsemi.com/products/timing-synchronization-sy...

- https://www.digikey.com/product-detail/en/microsemi-corporat...

What a time to be alive.

You can get refurbished rubidium oscillators from ebay for around 200$. Hooking these up to a Raspberry Pi should be fairly easy (see the pps_gpio kernel driver for that).

You should probably set up a true RT system if you're going to use an atomic clock.