Ditto everything OM said, however per your request places like Home Depot sell Tyvek Home wrap. I'm not sure if you can buy it in small enough quantities to be reasonable for you. Tarptent sells precut ground sheets (made from home wrap). You could see if one matches the dimensions you are looking for:

https://www.tarptent.com/store/tyvek-groundsheet

They also list the weights... they look to be in the 5-7 oz range so you probably won't drop half the weight.

You can also look into Tyvek kite material. A higher end, softer material. I think it might not be as waterproof as home wrap though, so I am not sure how it would perform as a footprint.