In reply to Gwinn512:
I would have a look at the alpkit bivvy bags they have a couple depending on how much you arer willing to spend.
For alpine climbing I would get the Alpkit Kloke which weighs 265g, its waterproof/breathable but not exactly plush as it doesnt have any way to keep the fabric off your face in the rain/snow. RRP £129.99.
If thats a bit steep price wise, you can get the Alpkit Hunka XL (the non XL version will be tight with a 4 season bag) which weights 500g, waterproof/breathable. Again very basic but it does the job. RRP £64.99
Hope that is of some use,