I use google cuz its the best search engine en everyone can acces .
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
USE EM
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 things and maybe u'll find something