The two aftermarket brands are Webasto and Inalfa. They each have local dealers/installers. If you buy a sunroof from a Scion dealer, they will use the sunroof sold by the closest installer to them. I had my sunroof installed after I bought my xB, used an installer that I was familiar with from a previous car, and they were a Webasto dealer.
So, you need to check out the Inalfa and Webasto sites:
Use the sites to find the closest sunroof dealer/installer. Ask the Scion dealer which they use. Wait for either Inalfa or Webasto to make a sunroof for the 2nd Gen xB. Decide which one you want, and whether you want to get it through the Scion dealer or go to the installer direct. I think it is probably cheaper buy from the sunroof installer, as I only paid $1,250 for mine. If you go direct, only use an Inalfa or Webasto dealer/installer -- I would not trust many of the pop-up installers. You might be able to find the sunroofs on eBay and be tempted to install yourself or take it to a body shop. I watched my installation, and I would only use an authorized sunroof dealer who had done an xB before and gave a factory warranty.
You want the biggest sunroof? Webasto made a huge canvas sunroof for the xB, and also made a Panoramic sunroof. My dealer refused to get the canvas sunroof, saying it has problems over the years and side spray from carwashes can get under it. And Webasto stopped selling the Panoramic sunroof, for xB because the opening was large enough to invite people to hang or sit on the edge, which deformed the roof and jammed the track. So big is not all good.