Pinnon Hatch Farms
https://pinnonhatch.com/poultry-sup...hicken-pen-fencing/poultry-protection-netting
But the size and type I ordered, and about the same price, seems easier to find on Ebay (same manufacturer - same seller - Pinnon Hatch)
https://www.ebay.com/itm/280937025834
Whatever you get should be knotted and strong, and probably polyethylene.
If you can find and afford it, the square knotted is easier to deal with than the diagonal knotted. When loose it drapes like a blanket and keeps an even shape. The diagonal wants to squinch together length-wise. The square knotted "holes" keep a square shape, and it all keeps a fairly straight edge when strung from corner to corner, so for sports netting it is easy and fast. The diagonal type holes naturally want to be in a diamond shape, so when strung only by the corners it looks like a rectangle with squinched middle - you have to pull the edges out to make the holes square and the edge straight.
Since for bird security one would have to secure all along the edge anyway, I'm not sure the extra cost (seriously more $), is worth it. I got the diagonal knotted as my first purchase to try, and spent a week cursing it, it is designed to catch things, and it will, even you. Then, you can stretch it out by the corners and the middle will be half the width. I eventually put it on a clothesline, to make a canopy with it, but as we know the hawk went around through a gap between the fencing and the netting. I knew then I had to make all sides and corners impenetrable with no gaps.
I found it best to tie it up with rags every 5 -10 feet, like a long bundle, and working from one end to the other. Having loose netting around, it catches on EVERYTHING, every little weed or stick gets caught up in it. So now that I know how to work with it I would get it again. Maybe 25x25 pieces, 50 feet is heavy to deal with. I cut it, but it is difficult to cut straight.
There is a great video online about choosing and how to work with both types, I will post it if I can find it, it is very explanatory. Basically running a rope along the edge, then securing the rope as you go, or working along one edge, securing every 2 feet at least, is how many people do it.