I own invitations for parties that I'm yet to have, just because they are pretty and I've picked party themes based on invites alone. I love invitations. Now my sounding board and source for custom invites, Amy, has gone public - Sadie Priss Soiree.
For years I've sent her my own design projects for critiques and suggestions. In fact this year, she hooked me up with a super sweet invitation based on my idea for a banner birthday festival for all three of my kiddies.
Amy has some really cool designs for invitations, party packs and sweet little posters. Check out her shop, her blog or even her boards on Pinterest. You might want to be prepared to have a party for no good reason other than you really want to use the invitation.