OK If I am going to get one of your awesome MP3 playing in dash car stereos, where do I get hte music?
#1 Obviously the best place to get mp3s is your own CD collection, rip them all to MP3 or WMA and off you go.
#2 iTunes and The Zune pass. iTunes has good points and bad points but it is the largest, most popular and arguably fastest. Zune pass has a financial advantage, Instead of spending a few $ each time you want to listen to a new song, spend $15/month and have unlimited access to all the music in the world. It is not much money and you can load up all your favorites in the first few months, and even after that $15 isn't much to have free music anytime you want it. Try new artists and songs you heard once on the radio without worrying about the cost. Nothing is more frustrating than hearing a song you like then dropping $20 on the album only to find not only does the artists suck, but the radio version you sort of liked isn't even on the album you bought. ARGH!
#3 Illicit downloading. Not that I would ever condone this sort of thing, but some people use torrents or file sharing sites to share music with others. Google the process or go to wikipedia to learn more.
#4 Direct sharing. Ask friends to throw some playlists or files onto a drive and share the old fashion way. It's a good way to expand your horizons and then you can go back and get the music the legal way using your Zune pass.