eBay (US Auto Nation), but I have a GX470. Not sure if the 100 series cushions are readily available. The one I bought is a non-OEM cushion but is M-USA and was a pretty close match to stock.Where'd ya find a lower cushion for $120? That's a deal!
Do you have any recommendation for grade/stiffness of foam to add to the bolsters?
We did use some foam on the cushion. The OEM lower had a upper foam pad that the aftermarket lacked. We used two layers of 1/2" craft foam from Wal-Mart. It was initially a bit harder than the OEM foam but has really softened up and now feels pretty nice. I think you could visit a Hobby Lobby, Jo-Ann, etc and find some suitable foam. I used Loctite spray adhesive to attach the foam, as well as make a few minor repairs in the remaining foam, which also worked pretty well.
FYI - my wife does quite a bit of sewing/crafting which made this job easier.