To answer a question within this string;
you can choose the location (Doctor, Store, Health location) and the supplier (Moderna, Pfizer, or J&J). Everyones geographic location may have an impact on what your choices are though. If you are patient and willing to drive a distance, it may open up your options. Likewise, if you want the first available and conviently close to you, the choices may by slimmer.
The key is, and doctors recommend, that once you get the first shot of the two shot sequence you need to stay with that vendor for the second shot; no mixing of Moderna and Pfizer)....