The International Society of Arboriculture provides professional training and certifies arborists. Arborists certified by ISA must pass knowledge tests and receive continued professional training to maintain their certification. Most arborists that are ISA certified will note this is their yellow pages ad. Also, the ISA website has a search feature that enables you to find arborists in your area simply by entering your zip code.
http://www.isa-arbor.com/findArborist/findarborist.aspx

Homeowners occasionally experience “sticker shock” when they receive a tree for service bid. Remember, as with most things in life, you get what you pay for. High quality tree work requires training, skill, experience, and proper equipment. Reputable arborists must also cover the costs of bonding and insurance. If you get multiple bids for tree repair or removal, look at the bid in terms of the overall package (site clean up, wood removal) rather than just cost.

Search Virginia, Maryland, West VA http://www.goodtreecare.com/search/index.cfm