Yes, the demo is still available. I even have it on my 3ds right now
The demo basically an introduction to the gameplay like how stuff works, what you can expect and other details. It is a separate app that you can download from the nintendo eshop. Think of the eshop as a 3ds equivalent of steam. Just scroll thru the stuff on your 3ds and look for the one that has the word 'eshop' in it. It's icon should look like an orange bag. Just tap on the search bar and type in 'sun and moon demo' and download the app that matches the same name. Once it is installed, play thru it
If your only goal is to get Greninja, then play thru the demo until you finish the trial, in which case you can find the professor in the Pokemon center and talk to him so you can send Greninja over to your sm game. I'm not sure if you can do other stuff after the trial but I do know that you can send the Greninja to the main sm game after finishing the trial so that should be your only goal
Important thing you have to remember is that the demo is separate from the sm game. You can only send the Greninja to your main sm game once and you pick it up in the Pokemon center in the main game. It's basically one Greninja per save file
