REI has some 2XL/3XL sizes that seem like they will work. Kinda pricey, but I see one for about $65. Do you have a local REI, Dick's, Eastern Mountain Sports, etc. where you could try one on? Or, order and return in store in case it doesn't fit?
One reviewer on this one says the L/XL fit her Size 18, 40DDD chest: http://www.rei.com/product/831135/mt...ns#tab-reviews
. Sounds like the 2XL/3XL would work for you as it fits 46-56" in chest.
Edit: I'd imagine this is something Cabela's may carry online, as well. They have a good selection of plus sized outdoor clothing on their website so maybe PDFs too?