The GTA San Andreas Ultimate is just a mod created by fans to enhance the basic version of San Andreas. Unfortunately, the mod has been released a long time ago which means you will have to use the resources available through Google Search in order to download it. I have already narrowed down the list. You just need to click on the link below to access it. There are also YouTube tutorials that teach you how to install the game, but as far as I know, there is also necessary to have the game installed on your computer as well.
Google Search: https://goo.gl/6xQ1cl