Google Flights in Explorer mode. You need to start with a specific destination in order to select Southwest as carrier. But then you can change it to "anywhere" and it will bring up a map with destinations and prices. Tue/Wed/Sat flights are generally cheapest and cheapest fares generally have a 21 day advance purchase requirement. You can also use "Flexible Dates" search in Google Flights instead of a specific date. As mentioned above, timing it with sales (like the current one) can help. RNO-LAS $29 fares indeed look to be cheapest WN fares out of RNO. PIT-MDW $46 fares look to be cheapest out of PIT.










One you have selected a destination, you can click on date field to bring up a calendar with dates and prices (PIT-MDW fares are shown below).