You can buy hardware and software specific for importing all of your old LP records into iTunes. This is a time-consuming process though, as the music needs to import in real time.
iTunes Music Store
With over two million tracks available for purchase, the iTMS is your gateway to a huge music collection. It costs 99c per song, and $9.99 per album. It's a great way to purchase music, with loads of exclusive tracks, without leaving the iTunes application!
Torrenting is illegal, but you can find many unsigned, legal for download albums up on torrent websites. Torrenting is a fast way to download full albums and discover new talented artists.
eMusic has thousands of songs available for legal download. They offer 50 free downloads. After that you pay $9.99 a month for 40 downloads (they have more expensive packages that lower the cost of each download even more). A pretty decent price, if you're interested in the non-mainstream tracks they have to offer. Amazon
Amazon has a download section of the site with thousands of tracks from unsigned and upcoming bands. They also feature cool remixes and mash-ups for download. Worth a good look at!
Download.com is the ultimate software and wallpapers on the Internet. Not many know they also have a great sub-section dedicated to music downloads. These are all legal and many of the tracks are from actual commercial artists who post exclusive studio recordings.