Lands End has some too - not sure how extended their sizes are, but when I was 250 the XL (18-20) was perfect. I loved their shirts. Plus you can return to a Sears if they aren't right.
Lane Bryant stores will also begin carrying maternity clothing later this month--I got an email about it, and I think they mention it on their website. I've never been pregnant, but I get all the Lane Bryant emails
I agree with osu2k1, Penney's had reasonably priced maternity clothes for me. I was a size 26 when I got pregnant the first time, and a 28 the second pregnancy, and I found nice clothes for a good price both times at Penney's.