[GraemeJ]

Have you heard of GoStand?
http://www.airturn.com/holders-stands/gostand

That's only the stand, you would still need the tablet holder to fit to it.  Looks to me as though there's little (if any) change out of $100) and I'm not sure if any of their tablet holders will suit a 13" tablet.

Graeme, if you would read only 1st sentence:

Well, I did - and I went to look at the GoStand.  However, the OP asked for something that would hold a 13" tablet and this means the GoStand would need a suitable tablet holder in addition to the stand. Hence my comment.
In fact, reading the OP, he was implying that it was the holder that was more important .... "but I'd also be happy to find something that sits atop of a microphone stand."