Any USB mouse will work on a basic level. You can buy them at any computer/electronics store and at most office supply stores that carry computer supplies. You should be able to get a simple one for around US$10.
Be sure to get a WIRED mouse. Most eMacs did ot come with Bluetooth and a wireless mouse that came with its own wireless adaptor cost more than you need to pay.
You don't need a mouse to transfer the music over. Just a keyboard that is an Apple USB keyboard.
Boot with the keyboard holding the T key on the eMac. Wait for the Firewire logo to come on.
Connect a Firewire cable between your eMac and your other machine.
Note the differences between Firewire availability on this image:
The top two Firewire ports are what the eMac uses. Some MacBook Pros have it, others only have Firewire 800 or Thunderbolt. For those that only have Firewire 800, you'll need this cable:
For those with 400, you'll need this cable:
Some MacBook Pro Retina models only come with a Thunderbolt port, that has the same Thunderbolt logo shown on the cable that adapts it to Firewire 800:
which you'd come with the first cable from macsales shown above.
This is far faster than any connecting you'd establish via ethernet.