Tyvek is a good lightweight tarp material. You can buy it from home improvement stores (it’s used as a house wrap) or sometimes get some free scraps from a construction site. You can run it through a dryer to soften it so it doesn’t crackle so much. It’s fairly waterproof and lightweight. You can also use a tarp. Either are much cheaper than the one you posted.

For a lightweight pillow, you could use some folded up clothing, or an inflatable pillow.