Subpoena his partners, their K1's, the partnership tax return for 2009. Also subpoena this same info from their CPA. If they have a bank loan subpoena any financials they submitted to bank.
Serve the partners so they know what is happening and get annoyed at your ex.
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In a Child Support hearing your ex should have to file and submit income and expense statements, just like you do. But you do have the ability to subpoena the records directly from the law firm, from their accountant, and from the individual partners.
You will likely want to request their tax statements, most likely with the K-1 attachments, but you can also request from the accountant the electronic records they keep in Quickbooks, Peachtree, or similar programs. Of course your ex may raise an objection about attorney-client information, but a lot of that might be able to be redacted.
If you believe that your ex has significant income he is hiding, you should definitely hire an attorney. If you don't have sufficient funds to hire an attorney, they may be able to petition the court to order your ex to pay for their fees. Please consult with an attorney to get legal advice specific to your situation, as this is just a general observation.