www.greatnortherndocks.com/about-us/ in Naples, Maine is the closest dock store with an outdoor yard piled deep with seasonal aluminum docks, where you can go load up the truck or trailer, and return with a dock project.
For the do-it-yourselfer, dock installer, seems like Naples, Maine is the place to go. So much for tax free NH when it comes to aluminum, seasonal docks? It sure seems it would be great to actually see it, before you buy it, as is the case with buying a dock on-line and getting it delivered via freight. Plus a dock store probably has close-out and seasonal mark downs.
www.centerharbordocks.com is more about custom made, big money projects, which they install, although it is a good local place to buy single dock repair items and pieces, after your seasonal dock gets ice crunched and you is fixing it.