For the ground sheet, how about some Tyvek? I ordered some on Amazon and it came with tie-off loops. I cut it to my preferred size (about an extra foot around my bag, but you can do whatever), put it through a cold wash to get the crinkle out, and finally ironed it with low heat. It worked great for me last weekend.
For the Tyvek sheet, I recently bought from somebody on Amazon selling a 4x8 feet portion of it. It comes with stake-out loops, so you cut it to size and then attach the loops. It's probably expensive compared to going to a home improvement store and buying a bunch of it, but the added loops are a nice touch.
Cut it to the size you want, and then throw it in a cycle or two in a washing machine with cold water and no soap. This will wash out the crinkle. I ironed mine on low heat to get rid of some of the wrinkles. Worked out well for me last trip.
You could pick up a bit of Tvek and use that. It makes really good ground sheets.
There are a couple of options: