Where do I look to find nonstop flight routes? I'm wanting to fly from Indianapolis or St Louis to San Diego on a nonstop flight. Thanks for any info you can provide.

Airport website

Thanks!

I live in Indy and the last time I checked (last month) there are no non-stop(direct) flights to San Diego. You have to layover (change planes) in all instances....

Thats right.And there will probably never be a non stop flight
to there from IND or STL.
Both IND and STL are mid range cities with limited direct flights only to certain Hub cities.
San Diego is a small airport, only one runway, with a limited number of cities that fly there direct.
Best bet would be direct from CHI.

A quick look using Kayak shows there is one nonstop flight each way from STL to SAN in July on Southwest. None from IND. I checked random weeks in April and July.

To find non-stop flights you use the airline search pages the same as for any flight, whether from an airline or an agency such as Orbitz or Kayak. Most if not all agencies and airlines specify whether there is a change of planes on the itinerary.
To airlines, by the way, "direct" means the same airplane from start to finish but it may make a stop en route. The search should make that clear but sometimes it is in the small print. Your choice of "non-stop" is a good one.