I've been looking also, but no luck as of yet. It looks like a phone call to Taurus is in order. Strange that there is no e-mail contact on their web-site, just a phone number and an address.

There are no extended magazines for the 709 yet. I think Taurus has enough other things in process that these aren't very high on their list.

Pearce Grips makes mag extensions. They don't increase the mag capacity but make a place to put your little finger. $6.35 at Cheaper Than Dirt.