You can get a Mac and Linux flasher from here: http://tablets-dev.nokia.com/maemo-dev-env-downloads.php
The flash image is not currently available outside Nokia but is inside the exe that you can download from the Nokia web site for updating the N9. Unfortunately this exe obviously only runs on Windows (I for example don't have any Windows machines around and for me the Windows exe is completely useless).
If someone manages to extract the flash image from the exe, it can be in theory flashed with Mac and Linux flashers. If you manage to succeed with that, please let me know and I will mention about it on my blog. Please write me at karoliina dot t dot salminen at gmail dot com or write to the comments on this post (please note that the comments are moderated to avoid spam and it will take time for me to approve them, so don't be afraid if you don't see them appearing immediately).
I tried to send a question about this to Nokia customer care [about how can I update my N9 with my Mac] but I was unable to do so because the form required all the lengthy numbers like serial number and my personal N9 was not with me today so I could not check it.
UPDATE: Got information that there indeed is a beta version (unstable) Mac Nokia Updater out there. http://betalabs.nokia.com/apps/nokia-software-updater-for-mac It does not state that it would support Nokia N9. I downloaded it and tried it with my N9. It didn't work, it stated that N9 is not supported.
Also people have been using a software called navifirm to extract the flash image out of the Nokia software updater exe to enable flashing it using Mac. However, according to my Googling this software also runs on Windows, so it may not be so straightforward for Mac users to use that route. Maybe the beta version of the software updater for Mac is the best bet at the moment in case it would start supporting N9 (that I hope but don't know).
It seems I still don't have a solution for this. If you know a working solution how to get N9 flashed with Mac, please let me know. If I will find a solution for this, I will post a new blog post with instructions how to do it with Mac.