"The best things in life are free."

SE Z550 themes:
http://www.esato.com/logos/colour/z550themes.php
http://www.myz550.lasyk.net/Z550i/Themes/

Other mobile phone stuffs:
http://www.getjar.com/
http://www.mobango.com/
http://www.mobile9.com/
http://www.mobiles24.com/ (this is the best site for downloading mobile games)
http://www.zedge.net/

..these are the best and these are all free.