Nowhere legally, I'm afraid. Previous versions are removed from sale when new ones are issued. If you have a legal, registered version then you could contact the Adobe help line directly, but I don't expect that this will get you anywhere - they generally reckon that the security of your download media is your responsibility. But that's the only place to try.
I would not be so definitive about the unavailability of older versions. I know for a fact that at least electronically purchased software remains available for downlad from the "My Adobe" page, even 3 years later. So the possibility definitely exists. Now I don't know Adobe's policy for replacements of CD versions, I would suggest you call Adobe before changing to others if it is the only reason - their policy will most probably be the same anyway. As a general rule, make sure to keep your software installation files in a safe place and make a backup of them, it's perfectly legal and much safer than just expecting someone will come up with a solution when you need to reinstall.
I know for a fact that at least electronically purchased software remains available for downlad from the "My Adobe" page, even 3 years later. So the possibility definitely exists.
So you've actually tried doing that, have you? I don't think you'll find that it actually works...
Adobe's policy is to remove previous versions of Audition from sale or download the moment a new version is released.
I did not now this English proverb, thanks for teaching it to me
If this means you don't believe me, I confirm that I went to My Adobe two days ago, and downloaded the AA3 I purchased on August 27th, 2009. Now if you still don't believe me, I think I'll recover
What I actually said originally - which is that Adobe remove from sale previous versions when they release a new one - is still absolutely true. If Adobe now give options to downloaders to retreive their downloads well after the event, well that's good - because they never used to, and it's caused a lot of grief in the past.