Trip to Basque Country from Seattle- few stops

I'd use Google Flights (though there are other options) with your home airport as the origin and first Bilbao, then San Sebastian, as the destination. Bilbao, as the major business center, may offer better connections, but you just never know. With the train itineraries from Madrid or Barcelona to Bilbao or San Sebaatian taking at least six hours, I think only flying all the way makes sense, even if it costs a bit more. Of course, if you also want to visit Madrid or Barcelona, you should check out multi-city (open-jaw) fares, which might be quite a bit cheaper.

When you've identified the best flights on Google, go to the airline website to make the purchase. Be cautious about using third-party websites.

Fares tend to bounce around a lot these days. If your trip is well in the future, you may benefit from watching fares for a while to get a feel for what constitutes a comparative bargain.

Note: I believe there may be a few bargain international carriers whose flights don't show up on Google Flights. I don't know whether Norwegian Air Shuttle, WOW, or Icelandair flies out of Seattle. If one of those airlines is available to you, it is likely that it will be your least exensive path to Europe, but you may have to purchase a separate ticket to get from your landing point in Europe to your preferred Basque destination. Figuring out appropriate timing for the second flight is tricky, because you'll have to buy a last-minute replacement ticket if you miss the second flight.

You might also check prices leaving from Vancouver. Sometimes savings can be substantial.

Maybe you could look into flying into Bordeaux, France and then a short bus or train ride to San Sebastián. I presume you mean the Basque country in Spain not France?

Madrid to San Sebastián or Bilbao. Paris to Biarritz or Bordeau.

Several airlines can get you from Seattle to Bilbao with just one connection in London, Paris, Amsterdam, Frankfurt, or Munich. Nobody flies directly from Seattle to Spain.

Bilbao is the largest airport in the region, plus the drive from the airport downtown offers a spectacular overhead view of the unique Guggenheim museum.
matrix.itasoftware.com will show many combinations, most involving two connections. Condor is the cheapest but it's along day.