I’ve largely ignored most of that collar advice stuff and just gone with what I liked and how it looked on me.
As for where to buy shirts, you’re going to need to be more specific, especially in regards to price. I mean, pretty much everyone sells shirts. It’s not hard to find shirts with button-down collars.
ADDING: thisfits and thinkingwins have some good suggestions and info (shoutout to the anon that remembered this).