Here is a very low cost supplier of new-manufacture “poly” sandbags: eSandbags.com . Regards, – TinCan
Here in Ohio, (and other places, I suspect), the feed stores formerly bought back used feed bags for 50 cents a piece. They no longer do. (If a used bag got bugs while on the farm, the bugs ended up going back to the mill.) All those good, heavy, plastic weave bags are now being thrown away. For those of us wanting cheap or free sand bags, just ask farms and stables on your area to save them for you. – Jim Fry, Museum of Western Reserve Farms & Equipment 
I’m a dude who used to live in Hurricane Central, Louisiana: Our city office was one of those sites where whenever tropical trouble or floods threatened, here would come the Public Works Trucks with a whole bunch of sand and the sandbags [to distribute free to local residents]. The funny thing is, they don’t make quite the same effort to go around picking up all these perfectly good supplies once the danger was passed.
Those white plastic sandbags do indeed breakdown in sunlight, but if you grab a bunch [before they do degrade] and store them away they will be okay to use at any time. Even filled with sand, and placed in a shed for storage, they will last a long time. How do I know? Did you ever fill sandbags? That is heavy, heavy work!So there is all this stuff that sat in the corner of our parking lot, free for the taking. First I placed a box of empty bags in my trunk to get them out of the sunlight. Then each day after work, I filled about four bags and took them home in the trunk of my car. About 20 sandbags a week.
My former property had a boat storage shed, perfect for lawnmowers & sandbags. Like the little Ant with the Rubber Tree plant, over time I acquired 300 filled sandbags–stored for emergencies.
And indeed these bags were used during the approach of Hurricanes Ivan and Katrina, while preparing my house. Damage was averted thanks to having these items on hand and kept ready.
Trust me, if you have to make 100 full load sandbags in a hurry, hen have an ambulance stand by while you are doing it! As with all preparedness, do it now while time is still on your side. – Clyde