some malls have stores that will "decal" or decorate mugs, shirts, and other things too with what ever you want on them.

you can also buy special clay and make your own, there are self drying and bake in the oven varieties as well as those that need a kiln. sometimes colleges have art classes where you can make things like this too.

you can also take one of the mugs from your kitchen and decorate it yourself with special pens called sharpies. (these are permanent ink pens in various thicknesses and colors like markers but dont wash off.)