We've used some that were recycled billboards or shipping container covers for years. Not sure I can remember the name(s), as we've bought from at least two sources. Haytarps.com, or billboardhaytarps.com, maybe?
Still have some that are pretty OK after probably 5-6 years or more of use... but the suckers are heavy to be pulling up & over on a regular basis. getting 'em secured so that they don't slip or slide off is a PITB.
Finally had a hay barn built, but still end up tarping 'overflow' from time to time, and feed the tarped stuff first.