Search the forums for "mule tape".
There are a few threads dealing with it. I found mine for sale at a flea market, got 140' for $cheap (5-10 dollars IIRC). If you know someone that works in the utlity trades (electricians, cable, etc.) they can often get it for free.
Muletape is used to pull cable thru the conduit. They often throw it away after one use. It may be a little dirty, but for free....