I use the CBD oil called Charlottes Web made by CWHemp. It is indeed legal and I do find it helpful (I use it as a supplement to a medical cannabis prescription - primarily a 1:1 CBD:THC ratio). There are many other brands available out there, but I have no idea about their legitimacy. The CW hemp company is the only one I trust at present. My suspicion is that too many other companies are selling snake oil, but there is little information available or independent analyses about the actual contents of what is being sold. Bottom line is to be careful and do your homework.
Dosage is normally determined by trial and error or so called self titration. I usually take 10-20 mg/ml per day using a 0.5 ml syringe. 10 mg/ml is probably a good starting point, but the rule of thumb is low and slow - start with a low dose and gradually increase until you find a dose that works for you. Since Charlotte's Web contains no THC (or at least a negligible amount), there is no risk of getting goofy.