how do I get_sidebar into a varaible?

I want to get a sidebar and save it in a Variable,
This is impossible with the get_sidebar function since it simply prints out
the sidebar, is there another function that I can use?

You probably need to use output buffering, if I understand your question correctly.

Try something like this:

ob_start();
dynamic_sidebar('sidebar-id');
$sidebar = ob_get_contents();
ob_end_clean();

That will put the output of the sidebar into the variable $sidebar, which you can then process and return from your filter on the page content.