I am not talking about junk merchant but is there a merchant that specifically sells large quantities of building supplies?

Not to my knowledge but some of the better general traders will sell things in forms of shipment such as "Shipment of screws - 25" or "Shipment of cement - 50" with the number representing the quantity, I've never bought any of these shipments though because they seem way overpriced and everything you need can be found in the world for free.

Yeah, some vendors will sell you a 'Shipment of [mats]' - the one near the Ranger Station often has some. Haven't found anyone selling shipments exclusively, tho.

Robotics Repair Depot east of Sanctuary constantly respawns mats for Turrets, btw - i hit it every day. Then on to Pac Flight 1981 and the Gunner camp for weapons to break down for steel & mods.