Ordered from Tupelo Honey originally. Only the front Oak carpet is still available for the front, Toyota Part # 58510-6A190-E0. It already has the insulating pads glued to the carpet. See post by @TNFZJ here: Time to install the new Carpet Assy, Floor, 58510-6A190-E0 - https://forum.ih8mud.com/threads/time-to-install-the-new-carpet-assy-floor-58510-6a190-e0.1371234/Where did your order your Mass-Backed carpet?
As far as buying factory replacement carpet, I thought it's NLA for my Oak/Brown color scheme...
I'm glad I saw your post. I was just about to order carpet from stockinteriors.com and upgrade to the "Ultra Plus Cut Pile" and they do have an option to add "Mass Backing" for $160 more.
The PO of my rig put some acoustic mat under the rear carpet area and doors. I'm going to bring that up to the front firewall and do the roof as well. I think the Mass Backing is probably overkill anyway if you've already applied some acoustic mat, but if it smells, then that would be a no-go for me for sure.
New back and rear fenders ordered from Stock Interiors: 91-97 Toyota Land Cruiser Carpet Complete Kit; Color:8655-Sandstone Plush Cut Pile. It is a little more tan than stock.
After the experience with the mass backing I would never try again. Mildly expensive mistake.