How to Get Mercenary Digi-Egg?

To get a new digimon, you need to get a mercenary digi-egg first which can be obtained by killing digimon. This is the picture when you get a mercenary digi-egg (the glowing one)

The egg will be looked like one of this in your inventory

There are many type of mercenary digi-egg, depends on how you get them. For example, you get a dragon digi-egg (far left on the picture) from a dragon digimon.