Find a video on you tube with the Dollhouse theme song: you could use:

http://www.youtube.com/watch?v=S2-zsU4twAY
(I know it is Buffy but it is the theme song for Dollhouse XD)

and convert it to MP3 using this site:

http://www.video2mp3.net/

happy downloading!

If you mean where you can find out which music is on Dollhouse then I made an article (Only season 1 but I will update it soon, honest! =P) but if you mean like ... to download and listen to on your ipod or whatever then you should get a livejournal because Dollhouse_music is amazing.