I love this tumblr blog. They’re pretty well-researched and you can send them a request if you don’t see your fandom or the character you want to cosplay 🙂
I have Dean Winchester as an example below:
Plenty of images of different pieces of clothes and accessories.
And texts about the outfits and links to where you can buy some replicas XD
I used this site to start my research for a Hogwarts student uniform. You should always do more research than just looking at one website, but this one if a good one to start with 😀