What do I do if I find hair in my store bought chexmix ?

Unless you have been harmed by the defect you probably don't have much of a case for personal injury or emotional distress and you may simply choose to write the manufacturer, which will generally replace the defective product and maybe even provide some additional benefits.  You should see a lawyer in person with all the facts of your case, promptly, to verify that this is your best approach.  Steve