[Koha] Obtaining MARC21 records

I have been playing with Koha some, trying to work out how it works so we can start using it at my school. My question is about sources for bibliographic records. What is the best (free) way to get a set of biblios to populate the catalog, so that we don't have to look up every item individually, either manually entering data or searching for items through the z3950 search? I see that subscriptions and ftp downloads are available from the LC, but the prices here are prohibitive. Is there anyhwere to download large numbers of records in large files to be imported into Koha?