I know of one Song Pack that's a CON file which I got from a OXM disk. I'll try to upload it sometime.
There's no other way unless you use the code provided on the back of either RB1 or RB2's Manuals to get a Song Pack from their website. You don't get to choose which one but they give you a large pack with a variety of songs.
There must be some way to by pass the Rock Band Networks Demo lock-out though. You download the entire song, but after about 30seconds in locks you out of it asking you to buy it. Hmm.. Only if I had modding skills, hah.
You go the the Rock Band Network store, Download the Demo. Then you go to Quickplay and there on the song list. You choose your difficulty then you play them like normal untill it pauses, asking you to buy it or delete the Demo.