I'll second the above comment that a ground sheet under the floor of a Tarptent is uneccessary. The silnylon floors are pretty tough by themselves.
That said, if you still really want one, you could fashion one yourself out of materials available at the hardware store. The plastic like the MLD ones is the same material as the window heat shrink kits you buy at the hardware store. Get a big one sized for a sliding glass door and cut to fit your shelter.
Same goes for the tyvek. You can buy the heavier homewrap style from the hardware store or order some of the lighter stuff like Max pointed out.
The tyvek is probably tougher material, but also heavier and doesn't pack down as small as the window heat shrink stuff.
Or, a final option… SOS makes a thin emergency blanket. It weighs like 2oz and costs $4. If its large enough (it might work for a small 1 or 2p shelter), you could cut a groundcloth from it and it could double still as an emergency blanket. This is what I use under my tarps for a groundcloth. Holds up about the same as the heat shrink plastic.