I'm using a converted camera bag, the Tenba Mini Messenger.
http://www.tenba.com/Products/Messenger-Photo-Laptop-Bag-Mini-Black.aspx
Simply remove the Velcro dividers and it will carry letter-sized files inside. It also has a separate padded compartment inside that will fit an iPad or an 11 inch MBA. It comes in 7 colors, and is very durable with lots of pockets for phone, chargers, etc. It also comes in two larger sizes, but if you want to stay small, choose the Mini. I have the black, gray (platinum) and burnt orange, and they are all masculine colors appropriate for an office.
I like it alot! I can pack my camera in there too!