Need help how to find easiest/ cheapest flight to northern Spain.

I am flying to that area in Spain for a wedding at the end of June, going from New York to Dublin to Santiago de Compostela. It was a bit better priced than going to Madrid first.

If you're willing to start in the west and work your way east, you can also look for connections into Vigo. It's not an interesting place, but it's quite close to Santiago de Compostela and I think it has more flights.

Look at Skyscanner.com and Google Flights for flights.
We found flights to and from Bilbao much less expensive than the smaller cities in the northern regions of Spain. You can fly LHR to/from Bilbao.

Im confused by your question. Have you looked on Google flights. I just found flights to Bilbao from DEN on Lufthansa through Munich in 13 hours with a 2 hour layover. Entering a random date for a 3 week trip, here's an example (if the link works):

https://www.google.com/flights?hl=en#flt=DEN./m/0htqt.2019-05-07*/m/0htqt.DEN.2019-05-28;c:USD;e:1;sd:1;t:f

Note also that because you are buying tickets now for May to fly in/out of a non-major airport, "cheap" is going to be very relative

Note also that because you are buying tickets now for May to fly in/out of a non-major airport, "cheap" is going to be very relative

This.

It's pretty late in the game if you are expecting cheap, easy and good. Pick one. Hope for two, but don't expect all three booking just 2 months out (and as indicated above, both in and out of non-hubs).

So...here I am "playing" travel agent.......

I selected a May 20th departure for you from Denver.
Here's what I came up with....

Norwegian Air Shuttle Denver to Paris (CDG) 8:15 pm departure from Denver - arrive Paris 1:15 pm May 21st $440/person
Air France Paris to Bilbao 3:45 pm departure from Paris (CDG) - arrive Bilbao 5:20 pm. $143/person.

These are their low fares.....no seat selection, limited luggage, etc.
So...it looks like you could fly Denver to Bilbao for slightly under $600/person with one stop.
(I used the Norwegian Air Shuttle and Skyscanner websites for this search.)

yeah you are going to have to buy separate tickets to get a reasonable price.

Look at Denver to London Gatwick on Norwegian

No website covers all flights. Through noodling around, I think that matrix.itasoftware.com has the most information. It's run by Google for the industry and does not sell tickets, but it appears to cover more than its sibling Google Flights. You can use the information to go to an appropriate booking site. You may benefit from a multi-destination itinerary, into one city and out of another, which must be booked through a multi-destination search function rather than individually.

If you decide to buy separate tickets (one transatlantic, a second into northern Spain), be careful to allow enough time to change planes at the gateway airport. Based on comments on this forum, some airports are (on average) more challenging than others. I recommend asking here about an appropriate layover time before you click on the "Purchase" button.

You may decide you need to go the two-ticket route because you see substantial savings, but be sure they are real after you add in the probable fees for checking baggage and factor in the possibility of stress if something goes wrong. The airlines will not help you out if you hold two tickets and miss the second flight because the first is late or canceled.

Also be sure that both flights use the same airport (not Heathrow/Gatwick or Charles DeGaulle/Orly).

Last November, United announced direct flights from DEN to FRA starting in early May. I'd fly United to FRA and connect to Bilbao (BIO) on a Lufthansa flight, because you can book it all as one round trip on the United website. Guessing this will be closer to $1500pp though.

Thank you for your responses. Part of the problem is that the only nonstops from Denver are to FRA, LHR, CDG, and MUN and not all of those are daily. LHR is by far the most price competitive and the shortest route in this market, but I couldn't find any nonstop flights to northern Spain from there. Changing planes twice is stressful. I know some of these add ons from discount airlines can add up, and I don't have any experience with ones like Ryanair or Vueling. I know I'm booking late, but it couldn't be helped. Maybe I should wait until next year if I can't get it close to $1200. I'll try these search engines you suggested.

Northern Spain is close to southern France, you could fly to Biarritz via MUN on Lufthansa, or fly to Paris and take the TGV to Hendaye at the Spanish border.