Kill repeatedly Digimon of the Keramon evolution line, they will eventually drop a mercenary egg.
Yokohama Village (lv7-16)
Snowstorm Village (lv27-31)
d-terminal dungeon B-2 (lv~70)
digimon Maze F1 (lv70+), F2 (lv81-84), F3 (lv82-85)
Infinite Ice Wall (lv>80).
You can also summon Infermon (champion stage) with Monster Cards lv5.