Old Navy always has the best "cruise" clothes. The stuff they carry is cheap, comfortable, and I know they have a section of their stores and website dedicated to women's plus sized clothing.
overstock.com should be helpful in allowing you to find the best prices for plus size clothes, and it offers an extensive selection of outerwear. With this website you can compare prices and the quality of clothes to find the best fit for you.
Here are a couple places you can buy plus size clothing: http://www.avenue.com/ and http://www.roamans.com/ .
Delta Burke has a line of plus sized clothing. Chadwicks also offers large womens clothing. You can request one of their free catalogs to be sent to your home. Many popular designers have plus size clothing lines. Calvin Klein, Ralph Lauren, Michael Kors, and Tommy Hilfiger all have stylish clothes for any woman's needs.
20% Off Womens, Womens Plus & Mens Styles
Macy's does offer a relatively large selection of plus sized women's clothing. They carry clothing lines like J Jill that offer very stylish clothes for the plus sized.
You need to watch your budget and find great tops for your mom. You need look no further than Trendy Plus Size Clothing: http://www.avenue.com/womens/clothing/trendy-plus-size-clothing
Size 18 onwards can normally be considered plus size womens clothing,
One of the best places to shop online for beautiful plus size clothing is Torrid. You can find everything from cute tops to pretty dresses. The website is available here: http://www.torrid.com/
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Most all of the plus size swimwear is already manufactured and ready to purchase. Some womens plus size swimwear is viewable at www.alwaysforme.com with many options in the swimwear section to look through.
"Not your daughters jeans, also known as NYDJ, does come in plus sizes for women. The tummy tuck jeans can come in sized up to 22 womens and 24 womens."
You're wife's a lucky woman to have a husband buying her new clothes. It's best to remember the style your wife likes and then search upon plus sized stores like Lane Bryant and Woman Within to find a great selection of swimsuits and colors.