Welcome, since you are a newby I will resist the temptation to sound like a smart *** and will answer you straight. I have never seen one for sale pre fabbed, I would say you are going to have to make it yourself.
I saw a picture of one on the web and it looked like a prefab. I guess it must have been
built by the individual. I haven't seen one anywhere but there.
corner cabinets are a pain to deal with effective storage UNLESS, you take a look at the lee valley system............worthwhile i say.............and build your corner up into an isalnd such as they do for kitchens.........just some more stuff to ponder..............and welcome