In reply to chris tan ( unVersioned): The Needlesports own brand ones are a particular bargain - choice of 15cm or 20cm lengths and solid (straight+bent) or wire gates all at the same low price. You may need to move fast to get them, though, as they were made by HB who have gone bust (I wonder why?)
Alternatively there's the Rock and Run ones made by Clog
http://www.rockrun.com/shop/type.html?d=2&t=11&sid=73af11087f028771... but the 10cm extenders are a bit too short for my taste.