Do you have an Omnibot 2000 robot - http://www.robotsandcomputers.com/robots/personal.php?filename=omnibot2k.htm - whose arm is just hanging limp at his side? More than likely, the problem is a broken arm joint. You can see where this common-failure part breaks in the second and third photos. I glued mine back together long enough to make a silicone mold from which I then cast a functional, albeit ugly, replacement years ago. Now that I have a 3D printer... why not use it for something useful?
This replacement is identical to the original with one exception - the top part is designed with more material to aide in strength. In the photos, I guess I should point out that the purple piece is the printed replacement whereas the grey piece is the original.
This is an impossible piece to locate and as far as I know, there are no other sources for either the joint itself or a 3D model. I am offering this to the public for the time being at no cost, but I reserve the right to pull this from the marketplace at any time. You are free to print this for your own use, but the sell of the file or the finished piece is strictly forbidden.
If you do find this model helpful, a tip would be greatly appreciated!