You could try www.dynamixmusic.com , www.powermusic.com , www.musclemixesmusic.com . They all have CD's of music at specific paces (steady state or variable speeds) and you can listen before you buy. Find out what pace you need by timing your footsteps for 15 seconds, then multiplying by 4 to get your bpm.
You could also do a web search under "walking music" or "running music".
I used to walk with some homemade walking tapes years ago: I just figured out the approximate bpm of the songs I liked, then drew up a play list of increasing bpm, decreasing at the cool-down for 60-90 minute workouts. That way, I had music I know I liked, and the pace I needed (most ready-made walking tapes were too short or too slow.)
I recently started back up walking, and since I got rid of my Walkman and my portable CD player a while ago, and don't have an MP3 player, didn't have any music to use. I found that I don't really need it: it's safer because I can hear what's going on around me, I have my Garmin to make sure I keep up the pace, and I have enough 'noise' in my head to keep me preoccupoied for 60-90 minutes walks!