I'm old enough to have worked with Coldwell Banker when they rolled out their commercial real estate operations on the East Coast (they had been only a west coast company at first).
What they did was assign a college student (typically on summer break or part time) to go out to each area and literally catalog all the commercial office space for part of the time and then the rest they would spend in the office calling up the building owner (or manager) and verifying the data collected.
This was many years ago but it worked quite well. A database was compiled and updated.
Bottom line is no matter how you cut this is a manual process.
You know why the yellow pages was such a success? Relevant data that while it came out 1 time per year typically was highly accurate for that year.
They made enough money (by selling advertising to a captive audience) that they were able to employ an entire quality checking and sales organization to verify and collect the data which ended up in a product that was widely used up until the internet came along. But once again they had the profit margin and captive audience to make the whole process work and that was a key part of that success.