If you are receiving support from him, the Army can assist you in locating him, and informing you of the command that he works for; however, the Army will not release any financial information to you without either his consent or a court order.
If you contact him and ask him to consent to the release of this information, the Army can make a third party disclosure to you of his pay, etc. (though the Army will not have tax returns).
If he doesn't want to cooperate, the only way you could get this information would be by securing a court order.
Alternatively, if all you need is gross pay and allowances for the financial aide information, you may have an alternative. Unlike a private company job, pay and allowance information is publicly available. The Army should be able to help you locate your ex-husband, and tell you his rank and duty station. If you know how long he has been in the Army, you can locate on the pay charts, located onhttp://www.dfas.mil/militarymembers/payentitlements/military-pay-charts.html and determine his pay.
This should give you the information necessary for the financial aide application.
Please let me know if I can provide any additional information.