If I understand correctly Adobe does not sell obsolete versions of their software so you would have to buy from other suppliers.
You could even buy second-hand, but in that case the transfer of the license needs to be registered with Adobe.
So look out and try to avoid offers that are not legitimate.
Why do you not want to buy Photoshop CS6, if I may ask?
I have the same question about installing CS5 into an additional computer. My old computers where I installed Adobe Photoshop CS5 have been replaced, and my Photoshop license only allows to install my CS5 in 3 computers. Now that I have a newer computer I need to install CS5 again, but why do I need to buy a complete new program? How did you resolve your issue from 3 years ago?