In Mass Effect, Where can I purchase Biotic Amp Interface X category and Omni Tool X category? Or can I only find it on missions?
The quality of items that you find, either from containers or from merchants, is based on Shepard's current level. Here is a base chart detailing which quality of items will drop at which level:
Rank I II III IV V VI VII VIII IX X Level 1-6 7-12 13-18 19-24 25-30 31-36 37-42 43-48 49-54 55-60
For standard containers, that is, containers that have no encryption, along with normal enemies, the above chart is followed. For containers that are locked with hard encryption, or for "special" enemies, a modifier of +1 may be added to your current level, and merchants may add a modifier of +2-4 to your level. Note: these modifier values are "for examples" and vary depending on the enemy, the container, and the merchant in question.
For this reason, it's often a good idea to leave Bring Down the Sky until later in the game when you have a higher level, as it will allow you to get high-powered omni-tools that are very difficult to find elsewhere.
Specifically, "X" rated items drop for characters of level 55 to 60, and are only situationally available from level 53.