I'm guessing you're after the cleaning wand that originally came with Mario Paint. We had the game through most of my childhood and I don't even remember having one, but apparently every copy came with one, so ours probably got lost or thrown out at some point.
I don't see any for sale on eBay or elsewhere, though I only searched briefly. If you're looking for one for actual cleaning purposes, I would think it isn't absolutely necessary and there are likely other effective methods to clean the mouse. (A quick Google search brought up a couple YouTube videos right away that may be helpful, if that's what you need.)
If you're looking for one for collection purposes, I don't know what to suggest. As I said, I don't even remember having one, even after seeing pictures of it, and I haven't seen too much about it online so I'm guessing it's quite rare. If you have any more info on it, I'd be interested to know more.
