What you need to do is:
Google something like "San andreas save game 100 percent"
then download a save game, you found, which has 100% completed SP.
Then get the file, open your documents folder, find and open "GTA San Andreas User Files" and then put the savegame-file into the folder.
Run San Andreas and you should be able to load the 100% completed SP saved game

Hope you understood it, feel free to ask
