Google maps is a great way to see other airports in the area. You can also simply google, airports near.....and enter your main airport. For me, I have a couple options. Portland is a short two hour drive depending on traffic. I need to also weigh the options on if its worth it to me. Bellingham (BLI) would be another option, especially for those in the Seattle area or north of Seattle.
Sometimes when I am researching flights, whether for myself or others, I like to spend some time doing just this.
Below are two international trips to Mexico:
The first one is from Seattle (SEA) departing on Monday and returning Saturday.
The second is from Portland (PDX) departing on Sunday and returning Friday
Now lets take a look at flying out of Portland.
For me to save the 11,232 points it is worth it to take the PDX option. I could almost book a second flight for the amount of points I am saving. The cash saving is at least $122.
Although I will spend a little more in gas over SEA, I am still saving enough in my opinion. The time of day I would need to drive would be at very off peak hours as well. Let's not forget PDX has some great lounge options via Priority Pass.
So the next time you are booking a flight, take a look at other options as well.