I just take as many little bits from as many bigger heaps (the freshest looking ones), until I have enough. I usually also do two samples. For fluke I treat all ewes, as most of mine have come from one place, and they came with it.... but I do have wet ground(just flooded recently again) right next to the river.
If you have a few of your ewes in pens, take samples of them, then some more from the ones outside. The vet will also advise. And empty margarine cartons (cleaned of course) are fine.