Only source? Not necessarily. What is the diameter of the wheels you have on the model now (excluding the flange)? By the way, 42" wheels might work also. Do you want just the wheels or should they be mounted on axles? If mounted, what overall length? How long a shoulder and what diameter? Seems to me you'd want an axle that's a direct replacement but with new wheels. I suppose all of this is my feeble attempt to say, there are many options out there just make sure you get exactly what you need.
Jay