Rapidshare is your best option.
Just google "invader zim episodes rapidshare"
and search around the links.
http://www.google.com/search?hl=en&q=invader+zim+episodes+rapidshare&btnG=Google+Search&aq=f&oq=
Here's some good links:
http://hotfilms.org/tv-shows-series/mu-rs-com-invader-zim-complete-series-130554.html
http://www.katzforums.com/showthread.php?p=560820
http://www.megauploadforum.net/viewtopic.php?f=37&t=9764
http://tehparadox.com/forum/f80/invader-zim-season-1-6-%5Brs%5D-9976/