Whatever floats your boat, man.
A very loose sketch of what you need in materials is:
For the SAVE THE DATE cards
" Cardstock, I used 3-1/2" in by 4-7/8" in cards (to fit)
" 4 Bar envelopes (4" by 5" envelopes see, they fit the cards!)
For the ACTUAL INVITATION
" Color-theme matching Paper (your safest bet is good 'ole cardstock. See photos)
This will include:
" Flat Cards (Smaller cards for your directions and whatnot)
" 8.5 by 11 (Letter) Paper, for putting the most of your directions, information & whatnot
" Envelopes (depending on the size of your largest piece of paper folded to your liking)
- This is where I used petal envelopes which I put into another envelope see pictures.
NOTE: Avoid glossy papers, they look nice... sometimes, but are easily scratched, guffed... as a photographer, they're nice to use but not for graphics. Most things look good on matte and luster paper. Also if you can, try semi-clear vellum papers, they serve as good covers for invitations... We will get into that later.
If it isn't obvious enough, I used paper source at www.paper-source.com as my major supply of paper. But there are tons of other ways too. The major benefit of working with a well established paper provider is that they have consistency in colors, thicknesses, and also a good variety of cuts besides standard rectangles.
But if you're fine with average colors (that is, colors that are close to what you are envisioning) go ahead and find paper off of.. ebay, or just paper you can find wherever you find most convenient. Just if you do, try and get all your paper (of the one color you get) specifically from one place, as not to accidentally get a purple from one place, and then get a puce-purple from another. That would not be good.
My photos also have been adjusted as not to give away the full names of the people I made these for (my sister and now brother-in-law) so the areas in which the names should be in, are a little blurry and crappy looking.