Usually the CRM or pipeline, the accounting or billing system, the job or project tracker, the support desk, the order or warehouse system, and the spreadsheets your team keeps beside them.
Read access or a scheduled export, whichever your stack and your policy allow. Every metric names one source and one refresh point, and both go in the scope document before the build.
If a number only exists in someone’s head or in a manual tally, it stays a manual input with a name against it. We’d rather show that than model around it.