Where can I purchase advanced biotic amps and omni tools?

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.

The best tools and amps are not sold in stores. You will never find a Savant amp for instance anywhere but in containers or as a quest reward (X57 Asteroid dlc for saving the hostages).