I usually do google searches to find good music. Another thing I do is go to my local radio station's websites and pull up the most recent "Top 10". I love listening to metal and hard rock (anything Metallica is good, though the older stuff is better IMO, Pantera, Drowning Pool, Megadeth)when lifting and R&B/Pop doing cardio for the tempo(Fergie, Pussycat Dolls, Rihanna, etc).
These websites give me good ideas:
http://au.blogs.yahoo.com/girlfriend...kout-playlist/
http://www.shape.com/playlists/
I also like to go to mtv.com and vh1.com and look through the music if you've got a few minutes.