You get wood at a lumber yard. Skills to build it unfortunately don't come from the lumber yard.

This is a mixture of branches / small trees with the bark stripped off (from the woods) and low-grade roughsawn lumber (from a sawmill). The materials are cheap, but finding a source for them inside the city may be impossible. I don't know the Boston area well at all, but I'd do the research to find a small sawmill or two within a couple hours' drive and then plan a day's venture into the countryside. Bring a picnic lunch.

Jon, thank you so much, that's super helpful!

If you can't find trees, landscape timbers might work. They have two flat sides (helpful for nailing and making the ladder) and are treated to resist rot.

You might also contact "WoodMizer" to see if they have a local customer in your area who could saw a log of choice for you. This would work for the roof. But be sure to choose a species that will weather well.