Do you mean something like these from Lee Valley? Cam Clamp Mechanism (1/4-20 Thread) - Lee Valley Tools
. If so they are one source but Essentra would also be a source and have a wider selection. If you are thinking of something different Essentra would be a good place to look for it. They have a huge selection of jig and tooling parts.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.