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I asked for a potato-free, corn-free substitute for the potato
starch in Bette Hagman's GF flour mix. The winners were:

1) just use extra tapioca starch (4 people)

> The reason behind this
> is that tapioca molecular make up comes the closest to potato starch.
> This comes to a scientific term as amylose to amylopectin.

2) substitute sweet/glutinous rice flour, available in Asian grocery
stores (2 people)

> It's basically rice
> starch & thickens similar to cornstarch although of a different consistency.
>  It does not have the goo/slim quality of potato starch though.  (It can also
> be toasted in a dry pan, then along w/ pan dripping & broth or milk, it does
> make a wonderful gravy w/ actual taste.)

I guess I'll try one or both of these.

Other suggestions were for arrowroot, bean flour, jowar/sorghum flour
(http://www.jowar.com), cornstarch (which evidently isn't corn-free, but
might help someone else out there), and a couple of people just
encouraged me to keep experimenting!

Thanks so much for your help.

Eleanor