Of course. You can make it look like any small creature. Cat, sparrow, octopus, falcon, spider... all are some of the things he has been. You simply have to redo the ritual every time you want to change your familiar's shape, which takes about an hour and a comparatively low cost of materials. ... Though not so low that it is easy to do all of the time, nor is it easy to consistently make that kind of money for most people.

Not to mention the high quality parchment and ink that is needed for copying sells, and materials that are not as easy to find as guano or cocoons. Being a wizard can get very expensive very quickly.