This is an example of a clock source which can be used with SDRs that will accept external clocks for e.g. pulsar observations – the recommendation was made by a member of SARA. I have posted the link here for my own information – I have myself never owned this device or seen it in action so can’t comment on how effective it is.
https://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=393
