I ran into the same problem. Here's some things I tried:
Of course, keep an eye out for sales, especially clearance sales. This is true for stores as well as catalog sources. Here are some catalogs & web sites I shop. All have fairly reasonable prices, and even stores that specialize in casual clothes often have separates you can use for work.
www.jessicalondon.com www.chadwicks.com www.lanebryantcatalog.com www.roamans.com www.lanebryant.com www.speigel.com www.junonia.com (expensive)
www.jjill.com www.justmysize.com www.silhouttes.com www.catherines.com www.eddiebauer.com www.landsend.com Another thing I have done with some success is shop on eBay. Often you will find new, unused clothes on sale, or things that are like new. I'm very careful about what I bid on, because it can't be returned. But some of my favorite transitional clothes came from eBay.
I have also "saved" quite a bit of money by SELLING clothes I no longer need. You can go the consignment route, of course, but I've sold mine on eBay. Because I buy so many things on clearance, and nice plus-size clothes are sometimes hard to find, in some cases I've wound up getting more for an item than I paid for it originally! I only sell things that are in excellent condition, and put in a complete description with a picture. So far I have sold everything I put up! That really helps with the clothes budget.