Ebay item231506402533
i use a basic ground sheet like this. Cost me all of £3 1mx2m in size. Quite robust and im not too concerned about buying a new one if it gets dammaged.
Poundland often sell that one, not a bad ground sheet at all.
yes i've been using that one for a while it's okay, like fluffkitten says it's only £1 from Poundland, it weighs 160g.