EOS firmware for a variety of Enterasys products is available at
How to Download Firmware for Extreme Networks Products
Note that service contract coverage may or may not be required, depending on the product and firmware version.
The B5-Series has a lifetime warranty, so does not require a service contract. You must login the support portal to access firmware. Login, go to Products page, Under Stackables, look for B-Series. Click to open and proceed to files and release notes needed.