Hi
Last year I was attacked by just about every pest you can think of, but the worst was the cabbage white butterflys. I bought some of the netting from the eBay link above, but it is very thin and tears easily. Also the mesh was very large and they just flew straight through.
This year I bought scaffold netting(sometimes called debris netting). You can buy that on eBay also. Its bomb proof, nothing has got through. The only problem are the one inch air spaces which stop it catching the wind. I simply tied them up with string.
I ended up buying 50mtrs for £40, but you can buy less.
The only thing that got my brassicas this year was slugs and I soon dealt with them and we are now harvesting cabbage, celabrese and lettuce.
This is where I got mine from
EBAY LINKIf you know a builder you might get used netting or offcuts for free.