Great Dragon Bank Edit
Possibly the largest one is the Bank of the Great Dragon, which was founded by Mammon himself, and is named after one of his titles. It is also called Great Dragon Bank. It was alluded to several times, for example as providing wealth for Mottom's Palace of Radiance. This bank has a railway line established on the outer city wall of Ashton, which operates with mercenary guilds that double as conductors and security (read: extortion and murder). How the Great Dragon Bank itself is managed is yet unclear, although it probably involves the Priests of the Count and may be managed from Yre and/or Ynamon.
Mammon himself has most likely delegated all banking business to his subordinates, as he is occupied with the Count.