Absentee watering

Please does anyone know where you can get the cone things which (I think) screw onto end of plastic drinks bottle with end cut off - fill bottle with water and it seeps down gradually? Think Monty showed one a while ago. Do they work? (should probably have asked that first)

 

Thank you, very Wise Owl. The reviews look good, and appears to be free postage, so will get a couple and see how they go.

 

I suppose it depends on: what you are trying to keep watered (e.g containers vs ground), how long you are going to be absent for and how large the water bottle is?