I just purchased the Staples brand of medium metal binder clips in black. I have purchased them previously and find that they work use after use. I also buy Staples brand large and small binder clips because they all work very well and we have never had any problems with them.
Although I thought I was buying a larger size binder clip this seems like a good value for the amount contained in each box. They are pretty small in size but would replace a large paper clip with better traction on a group of documents.