Cabbages - net to keep the butterflies off. I think Debris netting is the net of choice (price-wise) rather than horticultural netting
Search eBay "
Scaffolding and Ladders" section for "Net"
Carrots need smaller mesh netting (I think) to keep the Carrot fly off.
Peas need netting to keep the Pidgeons off whilst they are germinating, and again when the pods appear (so can be more a open mesh)
Depending on what height you need, hoops made from blue underground mains water pipe should do the trick. For taller netting I think some verticals to support it here-and-there would do - drain pipe perhaps?