There was a time . . . when the answer would be "Are you kidding?" Time and accumulated experience now lends a measure of credibility to the process of domain appraisal.
An appraisal by a reputable firm offers guidance about the informed buying market, a projection of the amount that those with specialized knowledge, who routinely buy and sell domains, will be prepared to bid. It will be a range estimate. However, due to a domain's uniqueness (they're all one of a kind), domain appraisal shares in some of the same qualities as art appraisal. Specifically, there are collectors who "just have to have" a certain work of art - which will effect price - just as there are people/companies who just "really want that domain".
To find a reputable appraisal company I'd suggest you look for companies/individuals with the sales/purchase statistics AND the experience in the bargaining process. It's one thing to have access to data concerning the publicly reported domain sales (lots of people) and quite another thing to routinely participate in the process of "offer, counter-offer, sale and purchase" of domains - both reported and unreported sales.
By reputation, based upon the above criteria:
Why do you think you need an appraisal?