I think they are called "Sleeve Nuts", I found them on ebay, you will also need some m6 threaded bar.
Ebay link below.
http://cgi.ebay.co.uk/Set-4-Sleeve-nuts-bed-fittings-bolts-New-/310084033012?cmd=ViewItem&pt=Beds_Mattresses&hash=item483272d9f4
Mark Pemby
July 2010