Bookkeepers are your friends. They're generally people who help you manage documents related to taxes, finances, etc. They should be able to help (and answer) any inquiries you have whenever a casino asks you something as long as they have your info. Though I don't think you need to go that far, it's just a source of wealth. Pretty sure a bank statement is more than enough and should satisfy the casino. Unless there are some other sources then just use that instead.

If you really don't know jack, then yeah, look up bookkeepers. With the abundance of VAs, pretty sure you can get one pretty cheaply. Would rather find a local one though just so that they know the local tax rules of your country.