I'm pretty sure we used
Deer Netting from Lowes, each square the netting has is about 1" square, its cheap, and available easily. But do keep in mind we didn't use it to hold balls, just to protect balls from getting into our robot in inappropriate places.
Just as a reconmendation though, if you can afford the weight I'd reconmend going with a solid polycarbonate like lexan. You don't want your nets to absorb the energy from the gravity and cause bridging, ideally you want the balls to bounce off the material and move as far down as possible, as a team that has tested it this year and used netting to hold balls in the past (04) we experinced a lot of jamming using nets. Lexan is harder so balls bounce off of it and move around more, netting is flexible and absorbs a lot of energy which leads to bridging and jamming. And even if you have something to break the jams, you've still lost time.