Dedicated Kimono Websites
I have personally worked with this store when they came to Boston. The owner is knowledgeable and fair, and I give this store my personal stamp of approval. They're LGBTQ+ friendly and willing to help you with whatever your dressing needs may be. You can find various kinds of kimono, obi, dressing accessories, tabi, haori, purses, kimono fabric, etc. The website lists product condition and sizes. They operate out of Ohio (no surprise) and for now only ship to the US. But their website also has various information, a blog, youtube channel, instagram, customer gallery, and more! I strongly recommend you check them out! I wrote a whole blog post about them here.
Kimono Yukata Market
This site has expanded over the years to sell a wide range of all the wafuku you can imagine. Men's and women's, furisode all the way to yukata. They have a rather large selection, a easy to understand quality rating system, lots of information about wearing kimono and measuring your size. At the time of writing this, they have a whole section devoted to plus size yukata with hundreds of items listed. Plus, they sell other items related to kimono, including storage containers and display stands.
One little note, I noticed on their Who We Are page they owners talk about being fed up of seeing fake kimono being sold online, so they decided to open up their own authentic kimono shop. Now that's an origin story we can all get behind!
Sou has an incredibly expansive listing of items. They sell kimono, yukata, obi, accessories, art pieces, tea ceremony pieces, and more. They often have some of the cheapest prices around, if you're looking for discount vintage kimono. They often run sales and have free shipping as well. This site is connected to Shinei and often the stores will list each other's products. Check their ebay for more items and even better deals.
Shinei is a great website for finding a wide variety of color and styles of kimono. Their array of synthetic iromuji is impressive. They also sell a large quantity of synthetic kimono of all types. Be careful, shipping can get expensive and isn't included in the price listing. One of the best part of sites like this in my opinion is the robust search and filtering system on the website.