http://www.gtainside.com/
this is the site to download cars you will get awesome stuff there (Y)
and regarding installation of cars and .txd files
firstly get the img tool from the same site and open the file gta3.img it will be located in X:\GTA San Andreas\models
after you open the gta3.img in the img tool you'll get a full list of all the .txd and .dff files files in the game
then all you have to do is that replace the .txd and .dff files in the gta3.img with the ones provided with the car you downloaded.
then all whats left is to replace the handling.cfg and vehicles.ide lines for the respective car by just opening them in notepad. they are located in the X:\GTA San Andreas\data
thats all and most of all don't forget to make backups for the gta3.img and the entire folder GTA San Andreas\data just in case you screw up.
cheers muaaah!!!!!!
damn i actually wrote all that.