Overstock.com usually has very good deals on duvet covers. Personally though, I don't use one anymore, as the down comforter itself always seemed to slide toward the bottom of the duvet cover and it was aggravating. I think that some duvet covers have fasteners that hold the 4 corners of the down comforter, but I didn't buy that kind.
Little known fact: down comforters are washable, even the ones that say "dry clean only". Dry cleaning chemicals should actually never be used on down, as they leave a film that will cause it not to "loft" as well. The better dry cleaners actually use the method I'm going to describe. There are a few tricks to it:
1. Use a jumbo washing machine at the laundry mat, the biggest one they have. I think it's called a triple or quadruple loader, and not all laundromats have one. I have a huge front loader at home, but it's still not big enough for my Queen sized down comforter that we use in the winter. I do have another thin down comforter that we use in the summer, and I do wash that one at home.
2. Use 1/3 the amount of detergent you'd normally use. HE detergent is best, since it's low sudsing. Pre-treat any stains with Shout, etc before washing.
3. Do not use fabric softener or fabric sheets...they will leave a film on the down feathers.
4. Wash with cold water, normal cycle. Do a 2nd rinse to ensure all of the detergent residue is gone. If you have the option to spin it again to get excess water out, do that too.
5. Dry on low. Be prepared...it's going to take several hours to dry thoroughly. Every 30-40 minutes, take it out of the dryer and give it a good shake. I also use a few clean tennis balls in the dryer, which help break up the lumps in the down filling. When you think it's dry, it's probably not...it really does take several hours unless you have a very thin down comforter. If it doesn't dry thoroughly, the down can mold. Do NOT line dry.
I use a flat sheet under our comforter to help keep it clean, but DH manages to tear the bed apart as soon as he gets in it. I typically take my bigger down comforter to the laundromat once per year, usually in the late Spring when it's starting to get warm. I'm there for a few hours, so I take a good book. Like I mentioned before, I wash the thinner down comforter at home, so I just do that one as needed.