How do I buy food in the US that I know hasn't been exposed to such pesticides?
You can’t. Best thing you can do is buy certified organic, which would preclude a lot of nasty agricultural practices. There’s no industry certification for pesticide-free. Certified organic allows natural pesticides, some of which are harmful.
Instead they use higher quantities of different nasty, outdated chemicals that were grandfathered. Repeated studies show that eating organic does not change the amount of pesticides found in consumers' blood. Organic food is not healthier.