While many millwork firms can produce decorative columns for interior use, not as many companies also produce exterior structural columns as well. Pacific Columns does both - as well as shutters, railing, balustrades and other related architectural knickknacks. Their columns are available in wood or composite construction, and you can get matching capitals in the full range of classical designs - use their neat web-based column builder to construct the perfect column for your backyard Temple of the Winds recreation, or perhaps something a bit more modest.
The same folks also run The Architectural Depot, an online "do-it-yourselfer superstore" with an emphasis on historic home remodeling and restoration. They carry an especially large range of wood corbels, tin ceiling tiles and the little bits and pieces that finish a project.