Typically, large carts are made by players for themselves. It is a rite of passage for a player to make their own cart.
Zyll sells wagons at Terra Noir. Find SW2 building around the perimeter of Moor's Rest, portal 1. Zyll also sells wagons and crates of support beams.
Skilled players frequently offer to build wagons for other players and outfit them. Ask in GL Freedom or Trade chat.