Start with the
FDIC's Internal Control guidance from their exam manual. The Federal Reserve has detailed internal control questionnaires (ICQs) in their commercial bank examination manual, which can be downloaded
here. The OCC and OTS also have ICQs in some of their manuals.
Of course, as mentioned above, COSO's
Internal Control - Integrated Framework is the prominent guidance on internal control. It can be purchased from
the IIA web site.
Sheshunoff, the Financial Managers Society, etc., have audit and internal control manuals that you can use as well.