I don’t know what access to hardware you have, but if I was you you would probably be better off making this yourself.

From what I’m understanding, you already have a hand cart/Dolly. I would advise that if you are not comfortable with welding to find a local machine shop most welders will be glad to pick up a simple job like this for $60 to $100 as you only really need a few welds to build out a basic frame on the dolly and extend the handles and casters to the dolly frame.

After you have the frame, all you need is a few 2x4s and plywood. Best