Any reason why you are looking for this particular mouse? I believe it was replaced by the 518, which is why you wouldn't find the 510 in stores anymore.
I use the mx510 mouse/keyboard combo too, and I haven't been able to find anybody that still sells it, besides on ebay. You can get em NIB for pretty cheap.
I have a red MX510 sitting on a shelf that is still in good condition. I upgraded to a Razer Pro 1.6 (great Woot deal) and have no need for it. You can have it for $20 shipped if you want it.
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