Legal Question in Civil Litigation in California

If you can't resolve it, then your remaining remedy is to sue. If under $7500, you can do so in small claims court, however, if you need to compel discovery and use subpoenas, then you should bring your action in Superior Court where those things a available to you. No amount of free 'tips and hints' from here or anywhere else are going to effectively help you in litigation proceedings. The defendant is likely to hire an attorney to defend.

If serious about hiring counsel to help in this, and if this is in SoCal courts, feel free to contact me.

You are going to have to sue. If you sue in Superior Court, as Mr. Nelson points out, you are entitled to use discovery. Additionally, you can have the court subpoena the bank records you need, and if you are represented by an attorney, the attorney can issue the subpoena.