You can try Sedar, Canada's equivalent to SEC.
With that said, there are different methods available. Though the following is primarily for the US market, it may be somewhat applicable outside the US.
First, companies (public or private) that earn a certain amount of revenue per year are often required to file forms with the SEC (Securities and Exchange Commission). Private corporations still have shares and shareholders, so certain transactions warrant filing of info with the SEC (i.e. Form D). I'd assume there may be similar controls outside the US.
Specific financial statements for private entities will be hard to come by. Now, even if you come across financial statements, private entities' accounting rules may be way out of whack, so it may be difficult to decipher useful information (try deciphering financial statements from public companies - even that's a difficult task).