I use Google because it’s de best search en’ine that everyone can access. The easiest search quote is “index of …”

Some kind of examples are:

index of ftp/ +mp3
index of ftp/ +divx
index of ftp/ +”whateveryouwant”

Google has many operators that should help you to specify your search. There are also lots of advanced operators available. Here are a few:

cache:
link:
related:
info:
stocks:
site:
allintitle:
intitle:
allinurl:
inurl:

eg:
allintitle: “index of ftp/mp3″

Try to combine thin’s and maybe you’ll find somethin’.