Where can I find an "accounting of all finances" form?

You have asked how to find an "accounting of all finances" form. I am not familiar with the exact terminology you use, but it sounds as if you are concerned that your brother is misusing funds and you want him to be accountable. If the only document he is using to control the money is a Power of Attorney, it may not be possible to get him to account for the money unless there is a court involved. I do not know of a state in which the court monitors the use of a Power of Attorney, which is one of the biggest problems with this kind of document: no one has to account to the court for how an elder's funds are being spent. The alternative is to seek the advice of an elder law attorney and find out whether asking the court to appoint a guardian is appropriate. Guardianship (also called conservatorship in CA) is fully monitored by the court and regular accounting is required. Unfortunately, it is expensive and time consuming. It is a last resort. I often say that a Power of Attorney in the wrong hands is a license to steal. If you suspect financial elder abuse, it is important to report it to your local adult protective services agency, or equivalent in your county. That triggers an investigation, at least. If you think there is misuse of funds, you can help the investigator or adult protective services by providing specific examples, such as missing funds from mom's checking account, dates of withdrawals, suspicious purchases by your brother, etc. You need concrete examples for such an agency to act. General and vague suspicions will not likely result in action.
Do what you can to confirm your suspicions. Ask to see the bank account statement. Make notes of your brother's response to this request, with dates. Ask him about her monthly expenses and keep records of what he says. It is very uncomfortable to report suspected financial abuse by a family member. If you want to protect your mom from possible abuse, it is important to act promptly and for her sake.