Well, you can try boutiques like Dorothy Perkins.....It's hard to define what is cheap....What about clothes from U2, their sizes do go up pretty large.
Or maybe
http://www.closetraid.comThere is this shop on the first floor of Thomson Plaza too. You can't miss it. It's facing Celia Low I think....anyway, you will definately walk past it if you are going to the food court.
I think the clothes there are more suitable for people who are more mature there but you can still find clothes that is suitable for office wear...just have to look.