Hi Theresa,
I went directly to McCalls/Butterick and they seem to have the pattern you want available. Here is the link for Butterick 6405:
http://store.sewingtoday.com/cgi-bin/but...p;amp;pageNum=1 This page will show you how much material you will need for the view you want. For example if you are a size 10 and the fabric is 45" wide, you would need 3 yards. When sewing with plaid you should buy at least a half yard extra so you have some wiggle room to match plaids if necessary. You will want to make sure that the seams all around this dress match on the plaid design. If you have never sewed with plaid before you might want to ask a friend or relative for some help when you are laying out and cutting the pattern.
As to fabric, the link below has some nice silk dupioni in plaid. I liked the Seville Mirage. It is very soft and feminine. The site has it on sale for $12.98 and yard and it is 54" wide. However you may have a fabric store in your area with another plaid that you like better. Just a suggestion.
http://www.warehousefabricsinc.com/Merch..._Code=SILKPLAID Good luck and please come back if you have other questions.
If you would like to share a photo and the story that goes with your big evening, please do and I will share that on the sewing page.