Hello!
Just curious, where is the helmet from? Reasonably priced and already assembled (commissioned) generally aren't synonymous with each other from what I've seen, as you are paying for the labor to put it together. I know that Crookknight (https://www.facebook.com/groups/679029939155041/) is now offering armor assembly services with measurements, but I'm not sure who else may do so. Walt's Trooper Factory has a great shadow stormtrooper set which is unassembled and Crookknight should be able to assemble with a cost. I think that RS Propmasters is still selling the fully commissioned shadow stormtrooper from the UK (http://www.rspropmasters.com/product/complete-shadowtrooper-commission/). I assembled mine myself, it came from Anovos, so I'm not too familiar with commission services.
Let us know if you have any other questions!