What single left handed keyboard do you recommend me?

The Half Keyboard from Matias does just this. You type with your left hand, and when you want to type a key from the right side of a keyboard you use the space bar as a modifier key. So to type an 'L' you hold space and hit the 'S'.

In addition to the Half Keyboard, which is literally half the size of a regular keyboard, they also make full size keyboards that can be used in this way. I bought a Half Keyboard years ago, for around $100. They now sell for $600, likely because they're making money on ADA-motivated purchases. Employers are much less price-sensitive in those cases.

I would try one of those keyboards that work with Layers. There are many of those, the one I use is the Dygma Raise (damn, I talk so much about it here that I am expecting someone to flag me as a corporate account any time soon). I think you can also look at Ergodox and Ultimate Hacking Keyboard, they probably support layers as well.

So the thing about layers is that once you hold the "layer key" (typically in your thumb), all the keys change what they do, and you can choose what it is. So you could have, for example, the first row be QWERT, but once you hold the layer1 key it changes to YUIOP, and then if you hold layer2 it becomes []\arrow-up. And so on. I could probably learn left-handed-only typing like that.