Where can I find firmware for EMC DS-300b?
If you've got a MyBrocade account, you should have access to certain FabOS images. Since it's an OEM'ed switch you may want to consult EMC support though, since each OEM have their own set of patch releases which they qualify (and typically provide to customers via their software download sites).
Hope this helps.
The FOS for EMC-branded switchs can be downloaded from support.emc.com but you have to have an account (and probably an active maintenance contract).