I had purchased an ADDAC Systems Muscle Sensing module to use in collaboration with a dancer.  But I needed more than one sensor, and so I also built my own, around a MyoWare Muscle Sensor and pickup cable, available from Amazon and other sellers.  Other parts:

Bud Case – Mouse 563-PN1323-DG

Eagle AAA Battery Enclosure – Mouser 12BH441/C-GR

Switchcraft 3.5MM TRS Phone Jack – Mouser 502-35RAPC2BVN4

Miniature SPDT toggle switch for power

After working a while with the dancer, I found that the MyoWare unit yielded a more useful range and behavior than did the expensive ADDAC systems module.  So I built another MyoWare unit.  Total cost: under $60 USD.  It is simple to build.  The three jacks are:  sensor input (TRS), raw output, and conditioned output.  These all wire directly to pads on the little PC board.  The mini TRS jack is intended for PC board mount and has no nut to secure it.  I used superglue to fasten it to the inside of the box.