For US stamps, I highly recommend Bill Weiss. He is extremely knowledgeable, his work is well-recognized, and his fee is relatively low.
http://www.stampexpertizing.com/Most people don't get stamps expertized unless they expect the item will retail $100++, as it usually is not economical otherwise.
Note that there is a distinction between getting something expertized (authenticity) as opposed to getting it appraised (value). I'm wondering if what you are really looking for is someone to appraise your items?