If I buy a costume in Tiny Tower, will all of my bitizens be able to wear it?
Yes you can! Once you pay the Bux to buy the costume, it remains unlocked and available to any bitizen you have in your tower.
I purchased the Astronaut costume for my bitizen whose dream job was the Planetarium, and have since put the other two Planetarium-dream-job bitizens in the same costume at no additional charge.