You don't need a ground sheet - you will be sleeping on a roll-mat, and your bivi-bag keeps you dry from below as well.
Any good outdoors shop will have bivi-bags - they're not complicated things. For us in Belgium, AS Adventure or Aventura are probably the best places (if perhaps a little pricey). For a basic good bag (must have breathable membrane, 'gore-tex' or equivalent), you will pay around EUR160+. Make sure the seams are heat-sealed with tape, and that there is space (particularly length).
I personally don't use the fancy ones with hoops etc. (if you want a tent, buy a tent!!), but I have colleagues who swear by them.
Sam.
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