if you still have your installation file and serial number, you can install.
if you purchased from or registered with adobe, you may be able to retrieve from the account you used to purchase/register, www.adobe.com/account.html
you can no longer (legally) purchase older versions of adobe programs including ps. you would need to subscribe to a current plan. the photographers plan is the least expensive way to obtain ps (along with lr and other goodies), https://creative.adobe.com/plans
It's 2020 and this is where things stand right now. CS1, CS2 and CS3 are dead & gone. The activation servers were removed from service years ago. Even if you find an installer, it's impossible to activate the software.
CS4, CS5 and CS6 are on life support. Adobe removed old installers. But the legacy software can still be activated providing you have the following: 1. original media (disks or downloads) you received at purchase and a way to read them, 2. a legitimate serial number for that version, language and platform (Win/Mac), 3. an older operating system that can support legacy 32-bit apps. -------------------------- NOTE 1 : Creative Suite installers won't work on macOS Sierra, High Sierra or Mojave without terminal workarounds. See the help article below. https://helpx.adobe.com/creative-cloud/kb/install-creative-suite-mac-os-sierra.html