Poor man's tarp can also be:
A piece of Tyvek. Comes in 9' by 100' rolls so the sky is the limit on size. It is tough enough for several uses and makes a good ground sheet. Use a good quality sealing tape to patch holes and wear points and you can get three or four camping trips out of one piece demoting it as it wears to less critical jobs. Last duty as as rock shield under the ground sheet.

A standard shower curtain works as a small tarp and, if sized right, as a useful rain cape.

Standard polyethylene sheeting, Visqueen is one brand, comes in black or clear and makes a good cover. The 4 mil stuff is okay for a couple of uses, a bit more if you patch any rough spots well with duct tape, but the 2 mil is pretty much one use only unless you get lucky. Black lasts longer in sunlight and doesn't act as a greenhouse. White, if you can find it, reflects a lot of heat and makes a decent sunshade on a sunny day.

Tightly woven nylons or Dacron, less subject to UV degradation, cloth can be coated with urethane to make your own tarps. Urethane coated and treated with silicone water repellent, Thompon's Water Seal is a favorite, it makes a tarp that sheds water well, lasts a long time and about half the cost of the commercial version.