Where do choreographers get their choreography ideas? 
There are many different ways to create choreography.

Some, like George Balanchine, get choreography ideas and create through a musical score and use the dancers to create musical visualizations on stage, while others may get their choreography ideas by using a thematic approach (pertaining to a theme. Ex: having a certain theme, then pulling out choreography ideas from there, or having a certain story, and taking your choreography ideas from there).

Personally my choreography ideas are usually through experimentation and improvisation.

So, how in the world do you get ideas from improvisation and experimentation? By improvising, remaining open, experimenting and allowing new movements and new choreography ideas to surface. 

You should choose whichever way is more efficient for you. Which one gets more choreography ideas flowing and surfacing? You will eventually find which is best for you when it comes to choreography ideas instinctively! 

Good Luck!