is there a site?
 

Is there a site out there where you could download old Nickelodeon shows/cartoons?

Go check out the torrents list thing in casual I did, I am downloading Rocko's Modern Life right now.

any of them will do?

Just tell us you wanna download Spongebob Squarepants. :p

Well try thepiratebay.org and mininova.org to start. If you can't find them there try the torrent search sites like isohunt. If it's not there, you can try looking through the non-signup sites but to be honest you'll be lucky at that point.