I saw a whole lot of them at Sam's Club last weekend (it is just a chocolate chip cookie the size of a med. pizza). But given enough notice any bakery that sells cookies should be able to do this for you
I've never had a Mrs. Fields cookie, so I can't say whether or not it is good, but I do know that they have gigantic cookies (pizza sized) and they can write things on them for you. There is a location in the Eaton Centre.