Unfortunately, it is going to be extremely difficult to find a strapless or even a convertible (or multi-way) bra in 54dd. Bra manufactures tend to emphasize support and comfort over versatility. That being said, there are still lots of pretty Bras available online in size 54dd.
Yes, you can purchase plus size bras on the following website: http://www.barenecessities.com/. They sell regular bras as well.
The store lanwbryant offers plus size bras on their online website. you can shop online to buy these bras. http://www.lanebryant.com/sexy-plus-size-intimate-apparel/4043/index.cat
The largest plus size bras come in size 55DDD. That is the largest I could find through online retailers.
Victoria Secrets does sell a wide variety of plus size bras. I would prefer you purchase your bras there. They sell lots of sizes and styles.
Quality Plus sized bras are available at Lane Bryant and Victoria's Secrets.
Plus size bras for women can be found at Penningtons, Laura Plus, Voluptuous, Shapely Figures, One Stop Plus, Addition Elle, Her Room and Ashley Stewart.
Goddess Plus Size Bras are available in a fairly wide range of different sizes. Specifically, the sizes available range between sizes 34 to 46, D to JJ.
Yes, they do make sports bras for plus size women. Just My Size has many different sizes and styles of sports bras made specifically for plus size women and women with larger breasts.
Cacique Bras normally are plus size bras. They come in larger sizes and are for larger women. They are also for women who just have larger busts and need a larger cup size.
There are many places to purchase plus size sports wear, including sports bras. One of the best online stores to purchase these is Catherines, which is a women's apparel store.
bravado makes some great plus size nursing bras. They are very comfortable nd easy on the eyes.
Shops that have great deals on plus sizes bras include Victoria's Secret, Amazon, and other general retail stores. You shouldn't have trouble finding a size 36E.