Often, I do not recommend one handed keyboards. Although there are many options for one handed keyboards, I have yet to meet an individual that is very successful with this kind of keyboard. The reason I find them difficult for training and use is that for an individual that was a typist prior to their injury or disability this is a new way to learn to type. This is a "chording" form of typing, holding down multiple keys to get the letters that would have been on the other half of a conventional keyboard.
As for mini keyboards,they can be very helpful for your one handed user. They have the standard QWERTY layout but since the keyboard is mini is decreases the amount of "travel" that the user requires for typing. There are many types of mini keyboards. Some are marketed to the disabled community, others are just "mini" keyboards, marketed to a more mainstream population. It is important to trial various types and sizes to find the perfect fit.
Typically, if a keyboard is for a disabled individual, it will be more costly. The keyboard below is $585.00, marketed towards individuals with disabilities.
There are also keyboards that are mini but marketed towards gamers. This keyboard below I love because it can be used with thumbs only (similar to a blackberry keyboard) and has a mouse mover as well. I have used this keyboard with individuals with varying disabilities. It has facilitated independence in all of the appropriate folks I have worked with. Since this is not for disabled individuals, it can be found online for $119.00