Filter products by availability

Hello



can somebody help me to set a filter according to availability?



I find how to set a filter according : " free shipment ", url=“http://kb.cs-cart.com/filter-free-shipping” [/url],but I do not find any info's how to set one for "in stock " products !?!



or maybe you know how can I set "Product fields " because are only 3 : price,free shipping, supplier .

How can I set another one: availability ?



thanks for your help