You could post a request for a firmware image in the
Kobo WiFi SD image thread in the Kobo Developer's Corner forum. This is likely to be the best route to go.
Check that you have private messaging enabled in your forum setup before posting.
For instructions, check the
Memory Upgrade Mod Guide (With Pictures) . The instructions are for a Kobo Glo but the procedure is not that different for any of the Kobo models with removable internal storage.