How can I send audio to airport speakers from a stereo system, not a computer?

I have some speakers connected to an Airport Express, and can easily send audio to them from a computer. So far, so good. However, I also have an actual stereo system (receiver, CD/tape decks, etc.), and I'd like to be able to send the output from that system to the Airport speakers as well.

I could probably do this by attaching a computer to the system, but it seems like overkill, and makes for a complex process (something like 'start computer, set up audio output', rather than 'push the button'). Is there some other, simpler piece of equipment that can take an audio input and send it to the Airport-connected speakers on my network?