As of 2/14/2014, Numrich Gun Parts still has M31 Drums in stock. Their price is $39.95 plus shipping.
Link to purchase M31 Drum
Centerfire Systems also carrys M31 Drum Magazines for $19.99 or $29.99 (depending on condition) plus shipping.
Link to purchase M31 Drum – Rusty Condition
Link to purchase M31 Drum
