Finding plus-size nursing clothes and bras is next to imposssible. But you can figure something out if you get creative. Keep in mind when you are searching a site that nursing bras are frequently listed under the maternity section. I found that wearing a camisole under a tshirt worked as a makeshift nursing top. You just yank up the cami and yank down the tee. It's a lot cheaper than a nursing top and works nearly as well. Bonus: get a cami with a shelf bra and wear it with your bra for added lift.
Here are a few sites where you can find nursing bras:
Motherwear. There plus-size section is actually pretty large, but they only go up to a 3X. I thought this style shirt was nice, this tunic (I will be singing the praises of the tunic for the plus mom later) looks pretty good, this cami is rather pretty, this embroidered tunic looks great for summer days in the park, and this 3/4-sleeve wrap top would be particularly flattering. For bras, the pickings are slim. They have four styles and go up to a 46H. Not of the styles are available in the full range of sizes, though, and they only offer one underwire option. My experience has been that soft cup styles do not provide enough support for me at a range of sizes from 40F-44DDD.
Bravado Designs. They offer nursing tanks (which have a built-in bra) and bras; their largest size is equivalent to a 46G. I own this one, and it is very comfy. The problem is that it's not the most, er, uplifting.Also try searching for Bravado bras on Google as they are sold on a lot of sites and the prices vary dramatically.
BiggerBras.com. Click on "style" on this site and choose maternity and nursing. This site has a wide array of bras, and includes a time-saving feature where you scroll over the picture and it tells you all available sizes. I spotted sizes up to 48H, and there may be higher sizes. They seemed to be having a big sale when I looked, so definitely check them out. Carry Goddess, Anita, Fancee Free and more.
Some more sites for nursing bras:
Decent Exposures. This is a strange little company out of Seattle that sells bras, underwear, nursing bras, and things like unitards. It's like a totally unhipster version of American Apparel. A lot of there stuff is made from organic cotton, and many items come in sizes up to 6X. I haven't ordered from them before, so I can't really speak to quality or sizing, but they seem to have very good customer service policies, so may be worth a try. Their nursing "UnBra" doesn't look that supportive, but it comes in sizes up to 54L, is made of organic cotton, and comes in a huge range of colors.
Pickles and Ice Cream. Most heinous name for maternity and nursing wear site ever, but carry at least one bra in a 50L. Direct link to that bra is here. They also sell bra extenders.
Ann's Bra Shop. Small but good selection of larger sizes.
MomShop.com. Terrible interface, but a few good bras.
For the crafty among you, there is always the option of converting your regular bra into a nursing bra. (Thanks to a member of Livejournal community Fatshionista for the link!) I discovered some of these sites by persuing Plus-Size-Pregnancy's Nursing Bra page. This page offers a wealth of nursing info for plus-size moms, but a lot of the links are out of date. Still worth a look, though.
Anyone have some nursingwear sites you want to add?