Need to buy some more sliding nuts with a spring for my rack

I have sliding nuts that screw on to threaded rods to hold the shelves in my rack. I want to split apart one of the shelves so I need four more of these nuts but I have no idea where to find them. The nuts have a button on the side so that they can be slid up and down the threaded rod quickly. 

Anybody have any ideas? I can't remember the maker of the rack.