I don’t know where you can buy them, but it’s super simple to make your own.
For the MU-02 (BlueTooth LE) model muse, you just need to connect a wire to USB pin 4. There are no other electronics required. Just a micro USB male connector, some wire and solder.
What you put on the other end of that wire for connecting to the body is totally up to you. The ones I’ve seen used by Interaxon staff have snap fasteners on, so they can connect to commercially available skin electrodes.