Hi, there are plenty of rubidium standards on eBay they are pulled from decommissioned cell towers.
You need to do your research they are not all the same. Mine is programmable (0-20MHz),
but unless you open it and remove a bandpass filter the signal is usefull at about 8-14 MHz if it sold as 10MHz.
Others are not programmable and stuck on the freq they are sold as.. which is ok if you wanted the specific frequency.
You know you've got a good one if it achieves a lock quickly after power up.
It should take only a few minutes to indicate atomic lock status.
edit.. re the ovenised Xtals, It appears GPS modules specific to timing are also cheap now, and I've gone that path.
There is currently no way to set this real time clock because a Human can't set the least significant digits for any future comparison.
Rather than lose time resetting the counter, I plan to pre-empt the next second and waste time setting that to the nearest instruction cycle.