Get it here if you have a valid support contract. This also means that people with a free developer account can download 6.5.21m. NOTE: Beware the installation caveats for 6.5.21m! Read the installation notes, which should tell you to install patch 5086 first if your previous irix version is 6.5.19m or older, or visit this forum thread for some more info.
BTW: IRIX applications CD and IRIX complementary applications CD can be downloaded as well with a valid support contract. To my knowledge this wasn't possible before.