There's not gonna be any site that is totally legal and offer everything for free. You either download illegally or you pay up for the downloads. Although, sometimes Visual Kei and Japanese rock bands do give out free demos. But, considering how alice nine. is now mainstream...your not going to find free demos.
Now if you want you can go to YouTube and use YouTube converter sites to rip the videos into mp3 formats. Though I cannot assure you if that's legal.