NB the suggested dupe has no working answers. note that the answers here so far are fabulous and work in 2018
I was just looking at flights from "North America" to ICN (Seoul, Korea); I observe that the price varies greatly depending on your departure city.
So, LA (seems to be) drastically cheaper than SF, Vancouver is different again, etc. The price difference can be very dramatic.
In some contexts it's possible to choose "where you leave from" (imagine you're doing a lot of business travel, or incorporating other visits, etc).
In fact, is there a site that finds the cheapest fares in this concept?
I really haven't found a QA about this on here
(I notice, the answers to fare-engine questions seem to change dramatically over time anyway. Would be good to know a solution for these days, 2018!)
So, "From which city in the US is the cheapest flights to ICN?" Or any similar "any part of country X to..." search.