You can get free indie mp3s from here
http://www.indie-mp3.co.uk/

But if you can't find anything there, try The Academy Is, ag Silver, Alice Cooper, Andy Paul, Ann Driscoll, Anthony Green, Anthony Hugh, Augustana, The Band that Saved the World, Barry Louis Polisar (he's more folky, though), Belle & Sebastian- I really like them, Ben Harper, Better Off Dad
Bloc Party, Blur, Brendan James, Bright Eyes, Cake, Cat Power, Coldplay, Coulours Run, Cordafonia, Death Cab for Cutie, Elly Kellner, Emma, Eric Nicholas, Feist, Fiction Plane, Finger 11, Five for Fighting, The Flaming Lips, Fred Savage and the Unbeatables, Frou Frou (If you like Imogen Heap, you'll like them), Iron and Wine, Jefferson Airpane, John Woodward, Keegan Dewitt, The Kate Rays, Kimya Dawson, The Kooks, Lanyap, Lily Allen, Madeleine Peyroux, The Moldy Peaches, Loons, Muse, Mott the Hopple (he's older), Moon School, OK Go, Pagoda, Peter Bjorn and John, Polysics (they're definately experimental), The Postal Service (The lead singer of death cab sings with them) the prayerboat, Radiohead, Radium88, Randy Coleman, Red Hot Chili Peppers, Red Sea Station, Regina Spektor, Renovaudio, Rilo Kiley (I really like them), Robbie Williams, Robert Steven Williams, Jack Johnson, Roisin Murphy, Rooney, Rosie and Me, Rufus Wainwright, Ryan Adams, Sanagi, Saosin, Sara Bareilles, Say Anything, Secret Suburbia, Senses Fail, The Shins (they're really getting big), Simian, Sonic Syndicate, The Specials, Spindle Cincinnati, Antsy Pants, Starrfadu (they're about to get big), Station Victoria, Beck, Sufjan Stevens, They Might Be Giants, Tiger Army, Tonic, the Ums, Underoath, The Used, VHS or Beta, Wilco (I really like them too), Within Temptation, Zsammy

I like more accoustic sounds, so I don't know if you'll like all of this. I go to Facebook, sometimes, because new artists will post their music for free trying to promote their items.