Does anyone know how/where to get more software instruments/sounds for Garageband? - Preferably for free...

I'd really like a sound like the synth lead at 1:20 in this song: http://www.youtube.com/watch?v=5bfseWNmlds

Thanks so much for the help! I really didn't know exactly where to put this thread so if it needs to be moved from the Pit I'd be happy to do that... Thanks!