This is always difficult to answer as I focus more on finding the article of clothing I want than where I found it. That said, I think online shops are the places I most consistently visit.
I actually just read an article from Business Insider about the Best 19 Places for Men to Shop.
Of the list, I’m only really familiar with 20Jeans and Combatant Gentlemen, but I’m sure you can find something good on that list.
Other online shops I enjoy are Aldo Shoes, Gilt, JackThreads, Tie the Knot by The Tie Bar, Topman, and Touch of Modern.
I’m also going to throw out two wildcard sites; Bombfell and Trunk Club. While I have yet to use either site, I love the concept and could see myself trying one out sometime.