Of course, there's always the SEC's own EDGAR database, and usually your company will post their current financials (at least) on their website. But some of the databases below offer considerable advantages ... for coverage of foreign companies, for sophisticated searching, and for downloading data as a spreadsheet.
WRDS (Wharton Research Data Services) is a data hosting and interface service available to academics only. Through WRDS we have access to several large datasets, listed below.
Entering WRDS: you will have to request a Day Pass. All this means is that you provide your eMarq email address, and then WRDS will immediately send you an email containing a link to the service that is active for 24 hours. If that is insufficient time, simply request another Day Pass.
For private companies, you're seriously limited in what you can hope to find. Because they're private, they don't have to disclose much financial information, so often all you can find will be sales or revenue estimates.
Both databases below include information on US private companies (and Hoover's has some international companies, though not many). Both will have estimated figures for either sales or revenue for some of these private companies.
If you're new to business and finance, some of these books may help ...