When I built my stormtrooper, I primed and filled it myself using a primer from my local auto store (halfords bumper paint for those in the UK)
Then I took it to my local smart repair place (car body shop) and got them to spray it. I had about 30 whites to choose from, they did an amazing job (used bumper paint obviously) and best of all? They were so excited by spraying a storm trooper helmet I didn't get charged.