Gas Cap replacement, Charcoal Canister location, P0446 CEL

I went to get my oil changed at Sun Toyota in Florida ($38.32 for Mobil 1, replaced filter and gasket - taxes included-Sunday special!) and the CEL came on on the way there. They were going to charge almost $90 to run a diagnostic, so I said no thanks and Auto Zone read (for free) a P0446 code which says:
Definition: Evaporative emission control system vent control circuit faultOp, and Probable cause: 1. Open or short circuit condition, 2. Poor electrical connection, 3. Faulty CCV vent control solenoid.

I have read in these forums that gas cap can be the easiest fix. However, I am not sure that is the issue, as it clicks tight and looks okay. Since the car has mostly lived in Vermont and has 212,000+ miles, there is a fair amount of underbody and rocker panel rust, and my muffler needs replacement.

I would love your input on the following:
1. Where is a good place to buy the cheapest new genuine Toyota gas cap for my 2006 Prius?
2. Where is the charcoal canister located and how can I get a precursory look at it and the hoses connected to it to see if there is anything obviously damaged?
3. Where should I look in response to #2 cause in the first paragraph, for a possible electrical short?
4. If I replace the gas cap or do something else that solves the issue, how far or long would I need to drive the car to see the CEL go away?
5. What other suggestions do you have for me.

Thanks in advance for your wisdom - I love this forum - I don't use it often but it has always been helpful!