+1 on the Marmot 800 fill bags. My husband and I both have Marmot Pinnacle's and we love them! They are always toasty warm! They are the best piece of gear we ever bought.
As for where to get them, REI has them marked down about $100 off a few times a year if you watch their sales. Or, if you have enough REI dividend money, you can pick one up for 'free' like we did.
There are probably many good options, but this was the warmest, most luxurious and relativly light weight bag we could get using our REI dividends. We have not been disappointed at all.