In addition to the 9-Euro ticket, there is another interesting offer on the German railways - the Egal-Wohin-Ticket, a train ticket which allows one to travel between any two places in Germany for 40 EUR.
I tried to obtain it in one of the Edeka shops in Bremerhaven, but when I asked for it, the cashier gave me a very strange look and said that railway tickets should be purchased at the train station. It was obvious that she had never even heard about this offer. I'm traveling long-distance within a few days and would like to obtain the ticket.
It could be possible that there are some limitations on where the ticket can be bought or the offer is no longer valid, or the tickets are sold out. If not, where can I really buy the tickets?
Edit: I tried multiple places where you're supposed to be able to buy these (Marktkauf, Budni, Trinkgut, Nah & gut, EDEKA) and in some Budnis, there were empty places where the tickets used to be saying that they are sold out.
Finally, I was lucky in one of the Niemerszein EDEKAs, but the process of buying the tickets was unexpected - they printed the code on a receipt at the cashier's desk (the one which serves also as info/tobacco/quick desk) instead of having a ready-made card for it.
I bought some and later (Wednesday evening) successfully booked a ticket for Friday morning which would otherwise be about twice as expensive. I wouldn't be so pessimistic about the slots in the trains being sold out - right now there are plenty of people who have the tickets available and I was still able to secure a slot, and with time the number of unused tickets will go down, and they are valid for more than a year.