It depends what you following. If you are NPK traditional gardener, then local extension just fine and less expensive.
Once you go into "alchemy" of biological gardening I have found best those 3 sites. They are slightly different in how they test. There also some informational articles on each site.
http://www.loganlabs.com/testing-services.html
https://www.aglabs.com/soiltesting.html
http://www.cropservicesintl.com/soil-testing/