Most banks use one of the standard payment processing gateways, rather than their own, so you won't see a mod for, say, 5/3 Bank. You'll need to contact the bank to see what gateway they use.

Manual credit card processing won't pass PCI compliance tests, so far as I know, which is why it was removed from Zencart. There is a mod in the Free Software Add Ons, but I doubt if anyone here who understands how things work these days will recommend it.

My personal opinion is that you wouldn't be doing your clients any favors by helping them set manual CC processing up. Better, I think, to explain PCI compliance and steer them to a real on-site processor (like authorize.net or Paypal Payments Pro) if they don't want to use Paypal Express or Google Checkout.

Another personal opinion - I think Paypal and other off-site payment processors are the wave of the future for small ecommerce sites, and Zenners (among others) need to get over their (often irrational) bias against them.