Undersink mounting brackets

[cgwalsh]

I have a granite worktop with a sink mounted underneath.
Recently i noticed that the sink was coming away from the worktop on one side & i have since discovered that the installer only fitted the mounting brackets on the other side. There are 6 holes for the brackets, but they only installed 2.
My question is where can i get some of these brackets from to install myself?
The bracket is a rectangular piece of metal around 40mm x 20mm with a slot down the middle & a 6mm bolt to fix to the worktop.

Thanks
Colin