I had a look around, and these are the bags I found which would suit my needs. The bivy doesn't need to be water resistant as I will be under the tarp, so waterproof is enough (I don't water resistance, though =). It should have good breathability and be light. Pretty much the standards, thus. Use is spring till autumn and in winter to increase the rating. Here the ones I am considering:
Titanium Goat Ptarmigan bivy
- 164 g in Intrepid/ Silnylon
- Mosquito netting, no zipper
- 90$
Rab Ultra Bivy
- 220 g in Pertex Quantum Endurance
- short zipper
- 65€
Integral Design Micro Bivy
- 535 g in eVent/ 70 denier coated nylon
- hood & draw cord, no zipper
- 210$
MLD LiteSoul Bivy
- 125 g in Momentum/ Spectralite .60
- no mosquito net, dual zipper
- 245$
MLD Soul Bivy
- 195 g in Momentum DWR/ Silnylon
- bug netting window, two sets of inside and outside sleep pad strap loops, top can be made in eVent
- 205$
- I also can imagine the Soul Bivy with a side zip
I know there's heaps more out there, and even a MYOG Tyvek bivy, like eg The Other Face has made, would be sufficient for my needs, though somehow I prefer to get it in slightly more durable material. As this is again a fairly new topic to me, I am curious to hear which bivy you use.