Since my boobs are in size flux, I thought it better to go to a lingerie boutique for a proper fitting and the opportunity to try on the actual bras before buying. The only downside is that boutiques tend to be small and thus carry a comparatively smaller inventory than the online giants. Lingerie boutiques usually carry a wide range of cup and band sizes, but they may not have your size in the bra that you want. Also, many people do not live in large metropolitan areas and their only options are mall stores with limited bra sizes, so the next best option is buying online.
Amazon, Bare Necessities, and Her Room are great places to buy bras online but with any online retailer, you risk dealing with the time and cost of returns due to inconsistent sizing and improper fit. However, if you have Amazon Prime, returns for Prime purchases are free. Panache tends to have consistent sizing and I had purchased the Della Bra at The Rack Shack in a 32H, so I decided to stick with Panache for my online purchases in order to mitigate the risk of having to return it. I had been eyeing the Maddie Balconette in both burgundy and spot/teal, so I ordered them from Amazon. I'm glad that I chose to purchase them on Amazon because the Maddie Bra in burgundy dug into my armpits and the cups were too small, so returning it was free, fast, and easy. Strangely, the Maddie Spot/Teal Molded Bra (see photo above) fit well, so I kept it. The matching spot/teal panties were out of stock on the three major American online lingerie marketplaces, so I had to order them on Ebay from a seller in England. Anyway, fabric stretching and discrepancies in the manufacturing process can cause slight differences in the size and fit of garments, so even if you are ordering the same style of bra in the same size there's no guarantee that they will fit exactly the same. This is why online ordering can be risky, but its saving grace is its vast inventory and ease of purchase.
Big brother is definitely watching...while I was searching online for the perfect H cup bra, I visited Bare Necessities and a few days later, I got a 30% off coupon in the mail from them. I used it to purchase the Marcie Balconette Bra in nude with matching panties. The bra looks nice but the fabric is a little loose at the top of the cup. However, when I push my shoulders back, that loose fabric is properly filled. The bra is not bad otherwise and it looks good underneath my shirts; perfectly rounded bust and nice lift.
Now that I live in NYC, I try to avoid buying things online because I have access to a lot of brick and mortar stores that carry almost every brand imaginable. But not everyone has that luxury. There are online bra fitting guides where you can size yourself and definitely make sure that you read a company's return policy before making your purchase.