If you own the original 2005 PC discs, you can install a 4.7 GB full game. Then, you can apply mods like SilentPatch (fixes bugs) and SkyGFX (modern visual effects). This is the version modders use.
If not free legally, purchase it at a deep discount. The game is frequently $3.99. Gta San Andreas Download For Pc Free 200 Mb