They're very close in dimensions, except the 10.1 is obviously quite thinner. The 10.1 is only 6mm longer and 3mm wider which are tiny amounts so any Xoom sleeve should fit fine, it'll prolly just have some wiggle room because of the difference in thickness.
Waterfield also has a slip in sleeve like the one your looking for, for the Xoom.
I just got a sleeve from Amazon today. It's for ipad and made by Philips. I would post a link but I have to have 8 post first. It's black and red and fits great. Plus its reversible and for $10, you can't beat it. Just search "ipad sleeve Philips"
Hey I have a sleeve I ordered custom made from Etsy, it came late so I had already bought the belkin one. So now I have a case and a sleeve that I don't need. I'll sell it to you if you want it. I've never used it so it's new.
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