how do I set the right side container to scroll in it's own place

how do I set the right side container to scroll in it's own place

So here's the link to my site. I wanted the right container to scroll in
place. How can I do this? I tried adding position: fixed to the right
column but then that destroys the layout. Right now the overflow always
scrolls all the way to the top.. Basically I wanted insta-shop-grid to
scroll inside that container itself.