Search under crystal in "Business Equipment/Test and Measurement" -- there are always sellers of HP TCXO's -- usually these are pulled from HP5328 frequency counters with option 10. Occasionally you will see an HP5334 with same. Lastly, some HP3586's (Selective Level Receiver) have a very stable oven - in this series the option is 004 -- all the above use 10 MHz. You can even buy a rubidium frequency source on EBay if you look frequently enough.
<em>Q. I’ve selected my synthesizer based on the output frequency required. What about
choosing the other elements in the PLL?
A. Frequency Reference: A good, high quality, low-phase-noise reference is crucial to a stable
low-phase-noise RF output. A square wave or clipped sine wave available from a TCXO crystal
offers excellent performance, because the sharper clocking edge results in less phase jitter at the
R-counter output. The ADF4206 family features on-board oscillator circuitry allowing low cost
AT-cut crystals to be used as the reference. <b>While predictable AT crystals cost one third as
much as TCXOs, their temperature stability is poor unless a compensation scheme with a
varactor is implemented.</b></em>
I have one of those 10e-9 ovenized sources that I run my counter and synthesizer from.