Asos maternity is your friend! They have loads of stuff in plus size, I just checked.
The quality is great for the price. I find their sizes tend to be on the generous side (though I'm not plus size so can't vouch for the larger sizes).
This dress is lovely and may be flattering for work:http://m.asos.com/mt/www.asos.com/ASOS-Maternity/ASOS-Maternity-Wrap-Kimono-Dress/Prod/pgeproduct.aspx?iid=5413772&cid=5813&Rf-300=1970,1993&Rf900=1465,4033&Rf-400=13510&sh=0&pge=0&pgesize=50&sort=1&clr=Navy&totalstyles=108&gridsize=3&un_jtt_v_frompage=0