Private Company Info in the US

I was reflecting recently on some of the ways to more deeply understand private companies in the US. Here is a summary of some of my thoughts. Be happy to discuss.
At one time or another I have found all of these helpful.

Key sources for private companies include:

1) Local press (local bus specialty press, microfiche local library, chamber pubs, e-zines, blogs/forums)
2) Subject matter experts (SME) - ex employees, suppliers, carriers, partners, customers, consultants, advisers (attorneys, CPAs, bus brokers, insurers, packaging material suppliers who are part of "integrated solution", bus park neighbors, supplier literature)
3) SBA, VC financing disclosures/records
4) Statehouse filings (principals, related names & companies, investors) and economic dev pubs (success stories)
5) Site visits - employment, busyness/shifts, carriers/suppliers, visible capabilities, some cities have traffic density data so you can estimate employment from number of cars, etc.
6) Directories - city, industry,chamber, etc.
7) Investment analysts covering an industry can have insight and networks into key private entities from their networks
8) Shows/events - local/regional/national, multi-industry
9) Adverts in local/trade press and SM presence
10) Building, zoning permits, tax assessments, EPA records, OSHA, water and air discharge permits, collective agreements & union officials...some of this under Federal FOI process
11) Local press reporters
12) What publicly traded companies they typically sell to are saying about their suppliers
13) Local academics/engineers/coops working as consultants
14) Local bus incubators, economic developers and their events
15) Personal networks
16) Bus/company/plant open houses
17) Food, beverage, hospitality, recreation establishments nearby key sites
18) Internal sales force needs to be enrolled and proactively directed
19) Off-shore folks interested in same (trade info)
20) Advocacy groups - e.g. environmentalists
21) Local charitable organizations they are involved with (boards, cont'ns, etc.)

Feel free to grow the list and elaborate any of the above from your experience or insight either privately or to the whole group.