Accurate breathalyzers available online

I just turned 21 and I was wondering if there are any breathalyzers out there that are accurate? I’m so paranoid of having a drink or two at lunch or dinner with friends or family and not knowing if I’m at the legal limit and getting stopped by police. I’ve seen some on Amazon but I’ve heard they aren’t very accurate. I’m sort of new to this stuff. Idk how long to wait after drinking to drive or how to even calculate a BAC. Thanks! Also please don’t be mean to me I’m just a baby…