The cheapest easiest way to get a super accurate oscillator these days is to use a GPS receiver. Rubidium oscillators from decommissioned analog cellular sites are relatively easy to find and also extremely accurate. I'd think there are very few applications where this is really critical though. In my experience, the RTC chips in old Macs are quite stable, drift of a few minutes per year seems typical. Since I don't generally use old computers as a means of telling time, I never found this to be a problem.