Purpose built one handed keyboards do exist but they tend to be super ultra expensive...and I mean like "unless your health insurance is paying the bill, don't even think about it" type expensive.
You might wind up with better value for money just finding a regular keyboard with a sufficiently powerful rebinding/macro/script system that you can assign keys on the right side of the keyboard to key combinations on the left side. Or maybe consider a regular keyboard plus a foot pedal to serve as a modifier. A good "normal" keyboard plus a good foot pedal should still run you a lot less than a one handed Maltron or Matias product.