I have a collection of B2/B3/C2/C3 firmware at http://alfirk.ccgs.wa.edu.au/firmware/
Note that you'll want to upgrade to 6.42 first, before going to 6.61:
https://community.extremenetworks.com/faqs-90303/i-g-c3-b3-series-upgrade-from-pre-6-42-directly-to-6-61-can-lose-configs-5791830
https://community.extremenetworks.com/faqs-90303/i-g-c-b-a-series-f-w-6-51-6-61-upgrade-considerations-5613291
https://community.extremenetworks.com/faqs-90303/c3-b3-series-has-ospf-errors-and-won-t-boot-after-upgrade-from-pre-6-03-to-6-61-firmware-5791866