In the case of drain flies that source is going to be some kind of very wet setting where a buildup of organic material is present. The larvae feed on this buildup that might be found in floor and sink drains, in sewers, in cracks and crevices on or at the edges of floors where filth accumulates, in A/C units, under refrigerators or washing machines, in bird baths and stagnant water outside, in sump pumps and grease traps, and nearly any place else that water and other wet organic matter can accumulate. In other words, there is no easy solution to the elimination of drain flies and to only use sticky traps for the adults is not going to be successful. 

However, these flies and others, such as Phorids and fungus gnats, present a great opportunity for you to add some needed services for your customer. You need to begin with a very thorough inspection inside and outside to determine what the source and sources are. You then make recommendations as appropriate for physically eliminating that breeding environment. This may be draining bird baths and plant catch basins and maintaining them better. It may be repairing leaks in equipment where water constantly drips out. It may be better washing of floors and allowing them to dry properly. It may be better maintenance of potted plants to allow the soil to dry or to cover the soil to add a barrier to the flies. But, it also includes the regular use of drain cleaners to eliminate the buildup of scum that the flies rely on. This can be done by you and it is an important service. 

So, sure, sticky traps and UV light traps can attract and capture most kinds of adult flies, but they will not control the situation. You have to go after the unnecessary breeding sites where the larvae are found.