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!