Filter products by availability


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

I find how to set a filter according : " free shipment ", url=“” [/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