You can find some scrap wood and make a sort of box over the needed area, then drape any sort of bird netting over it and clip it on. For cabbage butterflies, the small white ones, make sure the holes are small enough that they can't slip in. I believe Poundland sells suitable ones.
Or, if you can find some spare plastic waterpiping, either at a boot sale, or leftovers from a friend, etc, you can hoop these over the area by setting in some dowels into the ground and pushing pipe ends over these.
Another place to try is your local freecycle for bits and pieces.