Also check out:

www.sounddogs.com

www.soundrangers.com

www.pond5.com

These sites offer a la carte style purchasing as well as bundles. You can get some great foundational sounds here to expand upon. If you don't want to use Audacity, which is open source, then you could use Reaper which is $60 for a license if you're making less than $20,000 a year doing this stuff. It's a solid program.

Thanks,

Nate