We got our netting here, and have been very happy with the price and quality.
Diamond Mesh Nets, Lifeline - Netting - Seattle Marine
It's "knotted" netting, rather the woven, meaning there's a knot at each place where the lines cross. May sound odd if you haven't seen the woven kind but the first netting we got years ago was from a marine store (West Marine perhaps? I don't remember...) and where the lines crossed, the strands where woven together. It was more expensive then Seattle Marine, and much lower quality. I would highly recommend knotted netting over woven netting.
Like others have said, we don't rely on the netting, always use PFDs and tethers when appropriate, but I think the netting can help avoid some issues.
We've been sailing with our girls since they where 3 - 4, now 11 - 12. We've never (to the best of my knowledge) had a mishap where the netting "saved" them, but I still advocate for it and here's why...
Going forward in rough weather I've had a few times where the boat lurched and my foot missed the small side deck and hit the netting. Would I have gone over without it? No, I was holding on with at least one hand, perhaps two and wearing a tether, but I do remember thinking "That could have been awkward and painful if my foot went over the side there."
I think more then netting being something that's going to catch you in a free fall across the deck (which it's not likely too), it's more often going to give you those little nudges which aren't necessarily live saving, but do help avoid those minor issues.
How many times has that been a little nudge for my girls that I didn't even realize? Likely a few times.
I also like that now that they are bigger I'm happy to have them come forward and help in ever bigger seas as when the move forward and back, they can crouch down below the lifelines and be very secure from slipping off the side deck. Of course, wearing PFDs and harnesses if appropriate and holding on. They've also been taught that if the netting catches you, you did something wrong. Fix it next time. It's a backup, not to be relied on.