Where to get the end-of-episode pictures?
They seem to be bundled with the limited edition Blu-Ray releases.

For example, Season 2 Volume 6 limited edition Blu-Ray comes with all these goodies, picture courtesy of the official anime site:



On the middle right, you can see two postcards. These are the endcards from episodes 11 and 12 (the episodes published on this volume) free from any text obstructions. I found screenshots of the original episode 11 and 12 endcards on a Japanese blogpost to confirm.

The limited edition Blu-Rays have a suggested sale price of 6800 yen (about 68 USD) each, and it will probably cost more to import it to another country.