According to research posted in the article Save Your Bacon! on the Weston A. Price website:

Traditionally bacon was cured by adding sodium nitrite salts directly to the meat. Today’s manufacturers of “nitrite free” brands add celery salt, which is about 50 percent nitrate, plus a starter culture of bacteria. This transforms the nitrate found naturally in the celery salt into nitrite, which cures the meat. Although manufacturers label this bacon “nitrite free,” this method actually generates more nitrite from the celery salt than would ever be added as a salt. Indeed, “nitrite free” bacon can have twice the nitrite content of bacons cured directly with nitrite salts.

This means we should try and source bacon that is properly raised and traditionally dry cured, or cured with regular sodium nitrate salts for sausages and other cured meats. The best place to find these is through your local farmer. Many of them cure their own meats. If they don’t, often when you purchase a “side” of meat from a farmer, the butcher doing the processing will call you to ask how you want it cut, etc. At this point you can inquire with the butcher how they cure their meats. Most do it the traditional way without pumping extra junk into it like commercially cured meats.

Another option is purchasing from trusted vendors like U.S. Wellness Meats.