I see from your other post that you are going to teach bushcraft which is about using your initiative to use things for your advantage.
So, 1, Go to your local diy shop and buy a plastic tarp, very cheap while you save up for what you want.and will be useful as a ground sheet afterwards.
2, Buy some plastic sheeting and a roll of gaffer tape and an eyelet kit and make your own.
3, Buy some material and sew one yourself or get someone else to sew it.
4, Source a sheet - as Nomad said.
5, Fishing tackle shops have plastic tarps for a couple of pounds. I got one from our market to throw over my bike
when camping .