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I've got to say that I don't have any specific recommendations for vendors as I've never searched for that particular part - But one thing to consider is how much weight you can actually save by doing this, and "weigh" that in comparison to the amount of time/effort/money it may take to find, and then swap out, the aluminum sprockets. Unless you have a lot of them of your robot, it may not provide a significant reduction in weight (If you only have, say, 8 sprockets on your drivetrain). You may want to look into some other places to cut weight, such as switching to lighter bolts (or using rivets), or removing/lightening any nonessential parts, or using thinner materials. There are plenty of threads about other ideas for making your robot lighter (Like using the new(er) plastic pneumatics tanks, as opposed to the metal Clippard tanks -if you're even using pneumatics. That's just one idea though). However, if you do decide its worth pursuing to switch the sprockets, best of luck in finding hubs/sprockets that will work for you!