Well the first thing to learn is that not all cast nets are created equal, and by that I'm not referring to simply the size of them.
Go to Wall Mart and buy a cheap net and you will have just that, a cheap net which wont perform well and will frustrate you as you throw it. Ultimatly, it will probably gather dust somewhere.
Also, throwing a large net isn't as easy as it might appear, as they tend to get heavier as they get bigger if they are a decent net.
Buying an off the shelf net, even of good quality, might not perform well for all type uses. Catching Pilchards in 2' of water isn't the same as catching something in 10' even if the same mesh size is required. Sink rate which is due to the amount and size of the lead used around the perimeter is a major factor. For zeroing in on the bait schools of shiners seen on a fish finder, id be doubtfull you will
find a cheap net even worth having. Before you just go buy a net, do some research and be specific as to what your use for it will be.
We happen to be fortunate here in Florida in that there is lots of live bait used, especially by the salt water guides and fisherman.
Guides in the Keys either have it when they leave or they stop and net it after leaving the dock, and you wont be seeing any cheap nets.
Tim Wade from Melbourne FL. is about as good as it gets for custom made nets, and they are widely used by the guides.
And, they aren't as costly as you might think, and should you damage it later, he will also repair it.
A 6' custom net from him would be in the ball park range of about $125. I would call him and ask questions, especially of the need for large size nets. Don't forget, a 6' net is about 12' in diameter. 321 508 2836.