The ABTF subreddit is an amazing place to get help and information on finding your bra size and, as you might have guessed, a bra that fits. There, you can find guides on breast shape, how to measure yourself, information on underwire widths and a ton more. You can also create your own posts asking for more personalized help with the various stages of your particular bra journey. The ladies there are incredibly knowledgeable, friendly and helpful. It's a very body positive community that comes highly recommended!
Bratabase is a bra database where users of the site can submit measurements of their bras, reviews and give and receive advice. Users can create their own account and profile which allows them to keep track of their bra collection, body measurements, and much more. You can even sell the bras that don't work for you, via their listings page. As a result of that, it's also a great place to find new or used bras for sale at a (usually) much cheaper price than you might find with standard retailers. The listings page is a really great way to experiment with different brands, styles, cuts and sizes of bras you might not have had the confidence to try at their full price, for whatever reason.
BraSwap is another highly useful subreddit that allows users to create listings for bras they're looking to sell, trade or buy. Those bras can also range from new with tags to lightly or heavily used, and their prices will reflect that as well.
Bra Band Project
The Bra Band Project is another database that was created for the purpose of educating people on what different, properly fitted bra sizes look like. If you're one of the many people who is of the mindset that "DD is huge!", I would highly suggest hopping over there and educating yourself. Since cup size means nothing without it's band size, you might find yourself shocked at the size a "DD" actually is, compared to your initial opinion.
Big Boob Problems
While less of an educational resource than the previous links, ladies with "big boobs" might find they can relate with many of the users of this subreddit.
Small Boob Problems
The Small Boob Problems subreddit is similar to Big Boob Problems, just for the smaller chested ladies!
It could be a good idea to check out both "problem" subreddits, despite your own bra size, if only to see what the "other side" goes through. Big breasts and small breasts each have their own problems and perks. What matters isn't who has it easier or harder, but that we, as women, support each other instead of tearing the other down.
Listed in no particular order - some are based in the UK, some have free shipping (and free returns).
*I'm not affiliated with any of these businesses.