I'm a size 18 so I'm on the smaller end of "Plus size" but I've been able to find some clothes at Target,
Macy's and Motherhood Maternity. It's funny because the other day I was kind of wishing Lane Bryant had maternity clothes, I didn't realize they used to sell them! Since it's early yet for me I did buy some larger than normal tops at LB which I'm sure I'll have to ditch once my belly gets ...