This example shows these Data Alliance SKUs in the photo:
Radio: BulletM2-titanium (of course could also be any of the Ubiquiti Bullet series)
Mount: UBAM: This is adjustable so that the antenna can be pointed straight down from the ceiling.
Pigtail: Could use P4, P40
Other than the solution described above, we are not familiar with any omni made to mount on the ceiling, or mount made specifically for that purpose. However, we have panel antennas such as our A14 and A10R that mount easily on the ceiling, facing down, and send the signal all around a large building’s interior.
Another solution to mount an omni-directional like the A9N is to make your own mount from wood and metal tubing and paint it the same color as the ceiling.