AJAX autocomplete based on other field value
I know it's a bit amateur but the question has already been answered I just didn't find it.

It's here: 

form auto complete how to restrict results by previous drop down 

and here:

Using ajax to filter drupal autocomplete form

BTW: I would delete the question but since I didn't find the answered questions myself I thought that this one might be more discoverable. 

//I can't accept my own answer now, sory for that.