Have you tried half.com? I found it there for $12 on cd. ebay was also good at helping find it for even less. Amazon ran about $12 as well. I would check your library. I got the America Rocks! cd for $2 during their bi-annual used book sale.
My kids really responded to the visuals as well as the songs. The songs alone just doesn't cut it! I grew up with SchoolHouse Rock on Saturday mornings and I remembered most of the words, but more importantly I remembered all the visuals...a MUST for the visual learners in your classroom!