I know you can downgrade it easily but I don't know if you can get 1.0 in stores.
How To Downgrade:
1. First download this: I NEED 4 REED RULES LOL
2. Unrar/Unzip
3. Replace the original gta_sa.exe (C:\Program Files\Rockstar Games\GTA San Andreas) (BACKUP FIRST)
4. I believe you've just been downgraded! (I know this works for sure with 1.01 but not sure about 2.0. Give it a shot.