I'm trying to name ones that haven't been mentioned yet. Catherine's has local stores and online shopping and they usually have really good sales. Online purchases can be returned free at a store (the same is true for Kohl's and JC Penney). Sometimes I get lucky at Sears. Dress Barn has really cute work clothes and they often have really good sales. TJ Maxx is hit and miss, but cheap when you find something. www.OneStopPlus.com is fabulous for inexpensive clothing that is appropriate for work, and they have several sister sites. You can look for shirts according to length, which is fabulous! I need longer shirts since I'm 5'11" with a long torso, and I use this feature often. One Stop Plus is usually where I find the best sales. They always have at least one coupon code available on their site. Just a note: Old Navy does have some cute plus-size stuff on their website, but you cannot return it to a store. You have to pay to ship it back. You may also want to try www.Zappos.com. I don't know how high their clothing sizes go, but they have free shipping both ways, you can return up to 365 days after purchase if item is still unused, and they ship REALLY quick.
Good luck and happy shopping!