Headed to Koln: train or plane

Hello,

Last time I went to Koln I bought this one month open-ticket with my halb-tax abo where I can just take any train headed there at any time during the one month the ticket was good for. I bought it at the counter at Zurich HBF. For the life of me I cannot find it in the SBB.CH site. Can anyone point me in the right direction?

This is kind of urgent as I am going to travel within the next 3 days.
I don't know when I will return to Zurich, it's an open trip.
I have a B-Permit, and a halb-tax abo.
I will not be buying the tickets online.

Additional questions:

1. What is the cheapest kind of train ticket I can buy?

2. Would it be cheaper if I go by plane?

Thanks in advance.

*oh poop, this may be in the wrong sub-forum