These mines can be obtained from Elliott Tercorien after releasing him from stasis and going through the cryo lab with him. He will pick up some cryogenic parts in the area where Beckett and Daniels are frozen which enables him to start making the mines upon return to the engineering core. He will give you 3 cryo grenades and 3 cryo mines after you wait an hour for him to make them. After that must wait another 3 hours for him to make 2 more each time you ask.
The cryo mine was originally a weapon cut from the final version of Fallout 3 for an unknown reason. It does not exist in the final version of the game, nor does it exist as an unused item in the G.E.C.K. editor. However, the mesh and texture are contained within Fallout 3's BSA files, these files however were not used and new meshes and textures were made to replace them in Mothership Zeta (the original mesh and texture were eventually used in Operation: Anchorage as spider drones). Another cryo weapon, the cryolator, exists only as a mesh and textures in Fallout 3's BSA files.
Reverse Pick-Pocketing a cryo mine or a cryo grenade, then backing away from the non-player character will freeze the non-player character, while stunned you can pickpocket them at will.