Is there a merchant who sells shipments of wood?

Yes, there are 3 vendors that will sell shipments of wood.

• Connie Abernathy, Abernathy Farm (50 units)
• Supervisor Greene, Graygarden (50 units)
• Moe Cronin, Diamond City (100 units)

Besides that, the best way to get wood is by scrapping all trees, wood piles, stumps, and fences in settlements. Larger trees typically scrap for 20-30 units of wood apiece.

Moe in Diamond city has wood shipments for sale.

There is also a list here of things that can be scrapped for some wood

Although you could leave your settlement to shop for wood (or other supplies, like steel), I find it much easier to establish a Trading Shop inside your settlement instead.

The Trading Shop I built in Sanctuary sells "Shipments of Wood" in addition to other goods, including junk items that contain rare components like nuclear material, oil, etc.

Plus, constructing and manning a Trading Shop gives a +10 overall happiness bonus, and it generates caps. You also don't have to haul your massive mutifruit/purified water stash any further than across the street. It's basically a win no matter how you slice it!