Most costume shops like Buy Costumes, Costume Crave, Annie's Costumes carry the traditional Princess Leia garb in sizes up to 12 (they also have a plus size Jedi costume up to size 22) or plus sizes in the one size fits all which usually fits under size 18. The kit includes the dress, wig, and belt.
But truly the best plus size Leia costumes I've seen so far is the Star Princess from Big Gals Lingerie which comes in sizes 1x to 12x for $59 to $69 (depending on size). It includes the dress only and as you can see it is a little more sexy than the movie version. You would have to come up with some buns, accessory belt and boots. If you didn't want to be so leg bare you could cover up with a pair of white opaque tights from We Love Colors.
If you are not going to, or don't have enough hair to make your own buns Buy Costumes has fake buns for $8.99 as well as seven pairs of white boots from $26 to $55 including one wide width:
As for boots there are a couple of different styles at Zappos. I suggest Zappos because they will be better quality shoes (and have an excellent return policy) and you probably don't want to mess with your feet. Their prices range from pricey wide calf boots at $160 to more affordable gogo boot at $45:
If you have enough hair to do your own buns here is a great tutorial on Japanese buns that would totally work for Leia buns. When you get to the part about wrapping it around you could do it as shown or braid the hair first:
If you are looking for something last minute and a little more economical go with a baggy super white t-shirt, white jeans, and white boots (or shoes). Find a black space gun from the dollar store, do your hair up like Leia and you are good to go.
May the force be with you!