Using Rhapsody, Napster or another, with an ipod touch/itunes?

I own an ipod touch. I want to find a way to get large amounts of music (1000's of songs) onto my ipod with as little money as legally possible. I was looking into one of the companies that offers a small subscription fee, like Rhapsody or Napster, to download unlimited songs, put them onto itunes and sync to my ipod. Im wondering if this is even possible, Ive heard that you can only listen to songs for the subscription fee but not download them, Ive heard there is an app that allows me to put music onto this ipod touch, Im just looking for some information on whether or not this even works and/or how to do it. Many thanks in advance