Are you talking about the little piece that goes across to reinforce over the exhaust tunnel? Because when I was under the car it hardly seemed like a brace. Far too weak. The best way to find chassis braces might be to look for Ultra Racing Toyota Altis braces. In Thailand the Toyota Corolla was called the Altis, and they make a series of braces for them. I'd imagine all underbody ones would fit and work. They're expensive but: 1) work well, and 2) there's really not much else out there.
Here's the brace you were looking for:
http://www.ebay.com/itm/ULTRA-RACING-2- ... 5a&vxp=mtr" onclick="window.open(this.href);return false;
And here's a brace that I would say has made a massive difference to how the car handles:
http://www.ebay.com/itm/ULTRA-RACING-1- ... 1e&vxp=mtr" onclick="window.open(this.href);return false;
The one I recommend is a rear upper strut tower bar. You need to trim the forward trunk cubby in the Vibe to make it work (because the trunk floor panel won't fit without the cubby, and the bar goes right through the cubby) but it REALLY loosens the car up.
If you decide to give that a try let us know what you think. I might be interested in doing the same to my car.