Unfortunately, 2GIG does not sell the magnets individually. While we do not have any like magnets available, it might be possible to find one from another vendor online.
The specs for the magnet are:
Dimensions: 0.5 x 0.75 in. (1.27 x 1.9 cm)
The dw20R is meant to be used with the included magnet but this one should do the job. You would want to test it locally to confirm. When testing keep in mind the gap distance between the sensor and the magnet once installed.