I see alot of people on here looking for different size groundsheets for thier tents.
I am a contractor from Minnesota, this is how I create groundsheets for my tents and for some of my friends.
I side and have homes and businesses sided as part of my business. I have to buy rolls of Tyvek and other brands of underlayment for siding.
All technical (other than tarpaper), underlayment will also work as a ground sheet. These rolls of underlayment are sold at virtually every lumber yard. For a less expensive option get rolls of this material and cut it yourself using either a razor knife or a scissors. These rolls are readily available in 9' x 50' or 9' x 100' rolls for as little as $45.00 us currency a roll. In addition you can buy 2 story rolls 18' x 100' rolls but those need to be ordered.
In other words save money, cut only what you need and exactly what size you want and either sell the rest or save it or cut it for you friends..