https://www.ebay.com.au/itm/AUSCAM-MILI ... 3095469236Army hootchie. Not the size you want, and in a terrible colour, but plenty of soldiers (myself included) have slept under these in torrential rain - staying reasonably dry.
It is a cheap way to try tarping. You can also clip them together, if you buy more that one, to create a pretty large shelter.