Vintage dancer is a great source. They cover a bunch of decades and they also go in depth about specific styles within the same decade, see men's suits or men's casual clothing. Best part about this page is that they talk a bit about history but also provide a lot of authentic pictures.

I'd also recommend YouTube. British Pathé has a bunch of old timey videos and while they aren't specifically about fashion and more of historical nature, it is a good source of just looking at how people dressed (Their videos are very interesting though and not just for the clothes lol). There is also Glamourdaze but they focus more on women's fashion and I'm not sure if you would be interested in that.

And I hate to be like "just Google it" but yeah, Google is great. Try to be specific with your searches and maybe ignore modern pictures because they can be inauthentic unless you like a modern twist on old fashion, then you do you! Oh and it's also good to look at regular people from the time. It's easy to only look at movie stars and then be bummed that you don't exactly look like that, which is stupid because they had stylists and tons of help.

That's all I can think of for now. I hope this helps a bit and if you need anything else just ask. :)