Google has a variety of suggestions as to where you can buy a copy of Publisher.
BE CAREFUL of getting scammed!
My main point should have been if you already have, or can buy, any free standing copy of Publisher, then you can extend the years you use Publisher. If you have an older version laying around then use it...just do a custom install. If you have to buy a new copy, then any version from Pub 2010 to the newest should be compatible with the newest file version.
With that being said, the handwriting is on the wall. Publisher is going away...a free standing copy only extends the time you will be able to use it. It is time to start planning to move away from it. Personally I hope Serif or some other DTP company develops the ability to open Pub files directly before the deadline. But even if they didn't I would probably start transitioning by copy and pasting content from important Publisher templates and files while you still can.
DavidF