Most accounting firms are private. In addition to the sources recommended below, the Private Company Research guide can give you useful guidance for finding information about private companies.
These are business directories that can generate lists of accounting firms by number of criteria including: geography, employee size, sales, etc.
You can find company profiles for accounting firms in the following databases:
Professional & trade journals are useful sources of information for finding and updating information about accounting firms.
Suggested keywords may include the company's name, their parent company and/or their subsidiaries.