Well another last minute purchase. We've got a camping hammock for the woods, how about a bivy sack and nothing like 24 hours to buy one!
So here's a quick look at reviews from Outdoorgearlabs.com (they only review a few, but it is the base recommendation) and also looking at reviews on rei.com (sometimes useful), amazon.com (sometimes useful) and trailspace.com (pretty useful)
- Outdoor Research Alpine Bivy Sack ($240). This is expensive but get great reviews as Editor's Choice on Outdoorgearlabs.com and 4.5+ average ratings on REI and Amazon. The main issue is the cost and it is relatively heavy (900+ grams or 2 pounds). It has a hoop so the thing is not on your face. It has similarly high reviews on REI and Amazon.
- REI Minimalist. This is inexpensive at $100 ($110 for long), but it doesn't have a pole so it is right on your face and only a bug shield, so you need to put your rain jacket over it if it rains. It is a Best Buy at Outdoorgearlabs.com
- Mountain Hardware Bivy was on sale for $80 down from $180, but reviews said that it wasn't super waterproof.
In looking at some other sites,
- the Outdoor Research Helium ($170) looks like a good choice. It got a good review on Backpacker.com since it is only one pound and has the same design as the Alpine. ($163 at Amazon.com). It did have mediocre reviews at REI.com mainly because of internal condensation. This is a problem with all Bivy's because they are single wall, they will have condensate on the inside as they don't breathe that much. Shouldn't be a problem if you leave it a bit open.
In terms of where and how to buy, if you are in a rush, then REI has a store locator (that's how we got it). If you have a little more time, there are some good rebate programs out there. Moosejaw, Eastern Mountain Sports and REI all offer 3 to 1 or 6% rebates by click throughs, so check those out.