The Semco Model 120 gun has been discontinued by the manufacturer.
A suggested replacement for dispensing 1 ounce (37 cc) Semco disposable mini-cartridges or Semkit packages for multi-component materials is the Semco 250B Gun with a 1 ounce retainer.
The part number for a model #250B sealant gun with a 1 ounce retainer is 250010B
You will also need an air supply hose:
The part number for a 5 foot long Hansen hose is 280000