where can i download 10.10

If you are on 10.8 then your iMac must have 2GB of RAM or more or it would be stuck on Snow Leopard. So, the maximum OS you can install on it is 10.11 El Capitan.

You can get that here: https://updates.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg

If you are determined to use 10.10 Yosemite, you can get that here: https://updates.cdn-apple.com/2019/cert/061-41343-20191023-02465f92-3ab5-4c92-bfe2-b725447a070d/InstallMacOSX.dmg

Of course, I would recommend getting the most up-to-date version you can run on your hardware, and El Capitan was an incremental feature improvement update to Yosemite, and widely regarded as better.

Either way, you will need to make a bootable installer using a USB drive: Create a bootable installer for macOS - Apple Support

Apple dies not provide ISOs for download. They do provide the installer in the form of a .DMG containing an installer application.

How to download and install macOS - Apple Support