Are you talking about transitions with get darker in the sunor no line bifocals?
Just finished reading your question. At your age, you are talking about transitions with darken in the sun.
To answer your question, since most of your issues concern glasses and contact lenses, you are better served by seeing an optometrist.
The doctors who work at retail settings such as Costco, Walmart, Sam's are no different in training optometrist with their own office.
Most of these doctors are independent from the store side and should not care if you buy from them or take the prescription elseware.
So, for an optometrist, find a doctor you like and stick with him.
Concerning where to get your glasses done, again this is an issue of buyer beware. Private places generally do a better job than retail places, but if your prescription is low, shouldn't make a big difference.
Concerning contact lenses, this depends on the brand you are fitted with. With a good optometrist, he will give you different brands to try until you get the right one, usually no more than 3 brands are needed.
You can buy contact lenses anywhere, even online. Just make sure you go through a reputable and authorized retailer. Since they are pre-package it shouldn't make a difference.
Please remember to press the accept button so I can receive credit for assisting you today.