Thank you so much!
I get this question quite a bit, especially when I post character designs. So do take the time to go through my art advice tag if you haven’t already.
As for your question, there is no one or two or few places I get refs/ideas. I following clothing, history, film costumes, Reenactment and LARP blogs. 80% of the blogs I follow on here consist of this (as well as other things such as woodworking, glass blowing, dance troupes, blacksmiths, stained glass and TONS of other miscellaneous artists and crafters). I do everything I can to cast a wide net and constantly keep inspiring and interesting things on my dashboard since I scroll through it everyday.
I also have a pinterest blog that I use for archiving and have been saving/bookmarking clothing inspiration/reference for at least 10 years now. I own books on historical clothing, whether it be speculations based on artwork or archeological finds. I also read online articles, or watch video essays about the historical study of clothing.
I don’t limit myself to historical pieces either when researching clothing. I look to modern and walkway fashion inspirations as well.
To put it simply. I get ideas and inspirations from all over and I purposely seek out more and really dig into the resources I can get my hands on.
I do have an inspiration blog that I reblog things too if you want to see some of the stuff I’m archiving. I have a specific research tag where I try to gather resources. You can check it out here.
Claire Hummel (or Shoomlah) is also a fantastic with costume/clothing design and even created a masterpost of resources that you can check out here.