It is often best to work with a local firm so its architects can be on site quickly if problems arise. Ask friends and colleagues for referrals and try to choose people with experience in office renovations. Two online sources are the "architect finder" on the American Institute of Architects' Web site, at aia.org, and ServiceMagic, at servicemagic.com, which matches clients with architects based on location, budget, and a description of the project.
You'll need to interview at least two or three candidates to make sure you get along personally and have similar design ideas. It's important for the architect to understand your company, including its culture, hierarchy, and image. Make sure you fully understand the architect's fee structure, the proposed time frame for the project, and how cost overruns and delays will be handled. The AIA has sample contract documents on its site as well as a list of 20 questions to ask an architect, such as: "What is your experience and track record estimating costs?" and "Who from the architect's firm will I be dealing with directly? Is that the same person who is designing the project?"
For smaller jobs, you may be able to use an interior designer. But the designer will need an architect to sign off on any plans that involve rebuilding. Here, too, referrals are the best way to go. The Web site of the American Society of Interior Designers, asid.org, may also help. It has advice on finding a designer as well as a search feature that allows you to look for practitioners with degrees in design or professionals who have passed an ASID exam in your area.