The ones on diabetic warehouse don't work well enough (https://www.diabeticwarehouse.org/collections/continuous-glu...? Or is that site sketch for you? I think you can just get one if you are willing to burn $500 (not cheap, but only in the price range of a higher-end smartwatch)
Keep in mind that a single sensor will work for only a week or two, depending on the type. So it's not exactly right to compare the price to a smartwatch.