How do you find and upgrade the latest IOS software for Sup-720 MSFC3?

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We have an old Cisco 6509 with 2 x Sup-720w/MSFC3 running 12.2(18)SXD6.  IOS version is pretty old, and Cisco's software download section doesn't even have it in the list anymore.

We would like to upgrade IOS to the newest/latest, but we're not sure what the minimum IOS version that we need to put, in order to get to the latest IOS version.  Can someone please help?

Thank you,

[glen.grant]

  As usual any IOS upgrade is determined by how much nvram and flash you have in the supervisors . The latest versions take 512 meg of nvram and 512 meg of flash . These requirements are quite a bit more than for the old SXD train so check nvram/flash  capacity. Latest version for the msfc3 is

s72033-adventerprisek9_wan-mz.122-33.SXJ2.bin