Easy to find tires in the monocycle market for it is where 32" wheels are used mostly. Yet I find it is not a so good idea to build on such large size wheels, not with a hub motor for sure.
Large diameter wheels need a lot of energy, hard to accelerate and hard to brake. They do ride smoother, but they are a handicap to efficient handling and performance. I see them as a compromise, for someone very tall to cruise on brick roads for an example.