Slippery Slide [Where to get big blue plastic sheet?]

[Treefrog]

I think the product you are looking for , that’s found on building sites is called Visqueen 1000 gage. Which is normally sold in 2M+ wide rolls about 50- 100M long and costs about £12 a roll from someone like Travis Perkins.

^^^ What he said.
Unless there's a building site nearby and they have a big enough piece they'll let you have?