I'm going to go against the consensus-so-far (first four replies) and note that the O.P. wasn't really talking about water proofing a down bag, just making it somewhat water resistant --- and that this is very common, bags typically come with some sort of DWR (durable water resistant) finish, sometimes even with water resistant fabrics such as EPIC. In fact, I think it would be hard to find a sleeping bag marketed today that doesn't have some sort of water resistance.

Depending on how the DWR is applied or built-in, it's possible a person might want to renew it, and there are products to do that too. I use a single-wall tent, where condensation can be an issue, so I recently did this with my down bag using a product that you spray on and then put the bag in the dryer to let the heat sort of bake it in/on.

In terms of a bivy sack --- if all you want to do is prevent a little condensation, IMO that's overkill in terms of weight (and cost). And even very breathable bivy's can trap moisture inside, creating exactly the opposite effect you're looking for. I'm sure I'm not alone in the experience of waking to find my bag wetter for the bivy I slept inside than folks just cowboy camping under the stars with no bivy --- this can work either way, depending on the dewpoint, wind, local water sources, how much your body perspires, etc etc.

FWIW, the company that makes the down sleeping bags I own (one with a very fine reputation in the backpacking world), talks about renewing the DWR on their bags; they suggest ReviveX brand spray-on, which is what I used. You can read what they have to say about it here.