Any linear bearing will do that if not designed to keep them captive - customer of mine sent some Hiwin units back to use for us to send back to be re-balled (sorry, even the Yanks find that to be school-boy chuckle-ableCheap Chinese linear bearings might do the job but they are terrific at releasing the odd ball or two when threading the rod into the bearing. I have heard that you can live with a couple of balls missing but more than that could be a problem. Even Misumi bearings can release balls if roughly handled.
