What about some of daddy's old shirts or ties or hats? I remember as a kid we used my mom's and some of my dad's old clothes as dress-ups. Do you have any friends who work in the medical profession? We used to have fun playing with those footie things, caps, surgical masks, etc. that they wear. Maybe even some scrubs with drawstring waists? Heck, we even used to put my mom's old dresses on my littlest brother and thought it was fun. He did not care at the time and we have awesome blackmail pics.
cloak/cape (make a big half circle of cloth or just a rectangle, a button and buttonhole to fasten)
Sword and shield (cardboard would do)
Magic wand - a cool, strong stick or one from the toystore
Staff - a mostly straight, sturdy fallen stick of the right length, sand down pokey bits
Could be lots of stuff from the above: King or prince, wizard, magician, knight, old man, superhero...
Firehat, yellow jacket with reflective stripes
Doctor like pp said: white coat, face mask thing, maybe a real stethoscope from the drugstore.
Native American: fringed jacket, mocs, with craft projects
Cook: poofy white hat and an apron, toy(or real) food
Animals: floppy ears, horns, etc, clothes of fake fur
i have a ton of remnants i got at JoAnn and the girls just use those in various ways. we,of course, do have a ton of ballerina outfits, etc, like that, but they love those remnants. sometimes they hold them so they're wings or the make dresses or capes out of them. sometimes they cut them up for craft projects. i'd take the wee one over to JoAnn while your big guy is in school and see if anything speaks to you. sometime after halloween last year i guess i got this dark iridescent piece with faint silvery stars and moons -- perfect for a wizard's outfit. see what speaks to you for $1.75
We just had a star wars party and the kids wore tan tunics (easy to make-just sew the shoulders) and tied wth faux leather strips. They started as young padawans and then graduated to jedi knights. So cute.