In the U.S. you do need a prescription to purchase one.
You can just talk to your primary care doctor and tell them you're interested in getting one to screen for prediabetes, or to help you with weight loss. There's a good chance it won't be covered by insurance if you are not diabetic or taking insulin, but as long as you have the prescription you can pay out of pocket.
No, there are now CGMs that are available without a prescription as an assistive device for sports. You're not supposed to use those for medical purposes, but you can get them without a prescription.
You can just talk to your primary care doctor and tell them you're interested in getting one to screen for prediabetes, or to help you with weight loss. There's a good chance it won't be covered by insurance if you are not diabetic or taking insulin, but as long as you have the prescription you can pay out of pocket.