The first site is Old Navy. There is one page of activewear selections. If you catch it at the right time you can get some good buys. Tip: If you are an E-Bates member, you get 8% back at Old Navy.
Full Beauty has quite an extensive collection of sport themed attire. They have a special promotion right now. If you buy one, you get one free. There are some restrictions so read the disclaimer. On this site you will find yoga pants, bike shorts, swimwear, and jogging suits to name a few. You can also purchase footwear.
Lane Bryant also offers a nice selection of activewear. Most of their items are quite slick looking and very fashion forward. You should check them out if you are looking for new pieces for your wardrobe. They offer, jackets, cooling shorts, yoga pants, socks, and sport bras.
Now for the sites that pretty much specialize in activewear. Some of them include missy sizes, but all of them offer activewear in plus-sizes.
Fractal 9 offers leggings, shorts, and capris. They are beautiful and very colorful. Take some time to check them out. Pull down the Shop menu to locate the plus-size clothes.
Malesh Activewear is a site that only caters to plus-sized women. Their motto is "To offer quality activewear for curvy women at affordable prices." The prices are affordable, the offerings are limited. I believe this site will grow as people become aware of it. There is a blog connected to the site that features recipes and newsy information.
Lineage Another site that offers activewear. The sizes range from 0 to 30. Tops and leggings are available among other offerings. There is a blog on the site, but it has not been updated in quite a while. I do like the fact that they also have items for children. Although not as reasonable as Malesh, you still may come across some interesting finds.
Rainbeau Curves The first section I would hit on this site is the clearance section. You can pick up tops for $14.95 and yoga pants for $16.95 and they are mighty cute. Sizes run from 1X to 5X. The Blog on this site is worth reading and watching, because they have taken the time to insert videos. New arrivals run in the 30 to 50 dollar range. This site is a must!
Although there are additional sites that offer plus-sized activewear, I have chosen to highlight the sites listed here. If you think other web sites should be included here, please leave them as a comment.