Do a web search with that known search engine, just type "san andreas car mods" or something or along those lines. Im sure plenty will come up.
Here is one site i know of though
www.gtainside.com.
Just make sure you backup gta3.img which is in the models folder incase you make a mistake. oh and make sure that you dont change the handling of vehicles when installing mods, unless you want to be faced with a ban.