[c-nsp] Old IOS

[Michael Costello]

I have some old devices whose IOS I'd like to upgrade, specifically a
2511, on which I'd like to run 12.0S (e.g., c2500-k4p-l.120-21.S1),
and a 2916M-XL, on which I'd like to run early 12.0 (e.g., 12.0(5)WC4)
in the hopes of supporting 802.1Q trunking (the 2916M-XL is a 4 MB
switch, which will not hold newer 12.0 images).

The IOS Upgrade Planner has neither of these images.  How does one go
about getting his or her hands on old, unavailable IOS?

-michael