For the most part, Online Needlework shops charge a LOT more for DMC - I have seen it for nearly a dollar a skein at some, and it doesn't seem many put it on sale.
1-2-3 Stitch has about the best price of the online places I shop - 44 cents, most others are 50 cents or better.
http://www.123stitch.com/DMC_Six_Strand_Floss.html
Walmart currently has the best price of anyone, though as you said, I have yet to find one with the entire line; the only one near me that picked it back up only has 100 or so; I've been in there 10 times, adn have yet to find a single one I need, though, they also have no restocked it once since putting it out, and black & white were gone the first week

What Walmart carries is the same as anywhere else - DMC is DMC. I'm told by one of the ladies in my local group that another Walmart near her has a huge selection of them - it's too far from me to bother with, but good for her
Michael's & JoAnns & Hobby Lobby are all in about the same price range of each other - 35 to 44 cents, I think, depending on where you are, though JoAnns & Michaels do put it on sale every now & then. But Hobby Lobby is the only one of those 3, in my experience, to carry ALL of the standard line.
Some people have luck on ebay, but I am not a big fan of ebay, so don't have any sources there to point you towards...
If it is convenient for you, I'd suggest starting at Walmart and getting what you can, and then check out one of the other big box stores to fill in the gaps. JoAnns seems to have the most frequent sales on it, 3/$1, sometimes 4/$1.
I hope this helps!
I remember paying 10 cents a piece when I started stitching 25+ years ago, and remember sales of 20/$1!