WooCommere sort by specific quantity

Hey,

I can′t find how to solve this.
We need a sorting/filter function at the shop page, where the customer can fill in a specific stock quantity and sort the products by that. The same way as attributs like color and so on. I dont want to sort lowest to highest, just that specific stock quantity, so I can see the products that match that.

The page I need help with: [log in to see the link]