CS5 is no longer sold by Adobe. You may be able to find somebody selling the program on eBay but you take your chances with anything bought in this manner. If you are talking about getting a new serial number for an existing copy that you have then you can try contacting customer support Contact Customer Care . If you do so you will need to have registered the program originally and may also need proof of purchase. There is no guarantee that they will be able to provide you with a new number. Also be aware that if you intend to install CS5 on a new machine that it is not supported for any Mac operating system later than Yosemite or for Windows 10 on a PC.