TOPIC:

Hei there! I got a Kawasaki z400 from 1983. The previous owner managed to lose the original side stand. I am wondering if i can use a side stand from some other motorsycles? Is it the same side stand as the 750 for example? 

Is this a 4-cylinder motorcycle?  

Jes, it is

The Z400J 4-cyl wasn't sold in the USA, so most places aren't going to have parts diagrams for the model. 
Cmsnl.com is in the UK & lists a few Kawasaki Euro models, but I didn't see your motorcycle.
I would try the nearest Kawasaki dealer to see if they can help. 
If that doesn't work, maybe try contacting Kawasaki Europe (I'm assuming that's your location).

ps:  If I was going to guess, I'd say the 400J is similar to the KZ550 standard models or early 550 GPz models. just guessing.