I have product key for CS 5.5 but it was bought for polish version. Downloads available from Adobe pages does not contain polish language pack and with any other version I see invalid serial key error message.
This is a public forum, nobody here can provide a download link, and Adobe no longer supports CS5.5
For serial number programs you may download ONLY if you bought direct from Adobe via download -Log-in to your Adobe account and click this link https://account.adobe.com/products --click Order history on the left and then the link to your order that will show on the right --and finally click the link "Downloads and serial numbers" that will appear ---as always, write the serial number in a text file AND copy that text file and the ---download file(s) to an external drive (or two) to be sure you always have what you need -if you did not buy direct from Adobe via download you must find your original install -if you bought a disc and don't have a drive you will need to buy an external USB drive -if you bought and lost a disc Adobe does not provide a replacement via download -if you bought a download from a 3rd party vendor you will need to contact that vendor
Unfortunately CS5 is discontinued and unsupported. Unless you have your original installation media (disk or download) that you received at purchase, you're stuck. I don't know where you could possibly find another one. Sorry.
Nancy O'Shea— Product User, Community Expert & Moderator Alt-Web Design & Publishing ~ Web : Print : Graphics : Media