Quest Outfitters: tyvek Style 1443R.
1443r is lighter and more flexible than the standard contractor stuff, I got both and was sort of astounded by how heavy the standard tyvek was. Testing lightest to heavier, tried gossamer gear poly sheets, which are very cool and ultralight but they don't quite cut it for every type of spot, at least if one function of your ground sheet is to protect delicate materials on top of it, like silnylon floors or whatever. 1443r is next to test, if that shows weaknesses I'll go with the thick and heavy contractor type stuff. But I'm hoping 1443r, which is what Jim Wood recommends, does the trick for ground cover, it's a lot nicer to deal with than the standard type, which is stiff and relatively heavy.
Quest will allow adjustments to the order, so if you order 3 yards, then in the final comment box on the order form, last step, say you want 2.5 yards, they will adjust your order price after, and also maybe the shipping, so you can get exactly as much as you need.