I would suggest beginning with the ISA website at http://isa-arbor.com or the PNW-ISA website at http://pnwisa.org. They both have great resources and lists of local certified arborists.
Oregon Tree Care is a great place to start to search for a local aborist company. Give them a call and they will come give a free estimate. They are young, experienced, and have the muscle to get the job done!