Check out Jasmine and Woo's "Trick or Treat" invites available at many fine stationery retailers (like FineStationery.com HERE).
Per their description:
Have a spooktacular Halloween party with this fun invitation! The words "Trick or Treat?" are preprinted in black and orange on the left, with your party details printed on the right. Black and white stripes and an orange and a white polka dotted band accent the bottom of the card.Want some extra STYLE POINTS?
Order the black and white bat-silhouette envelope liners too.
The invites are little pricey, starting at 20 for 50 smackeroos, plus an extra charge for the liners, but again--- STYLE POINTS for the bat lining.
Hmmm... I feel like I've heard that on the '60s Batman TV show.
"Robin--- TO THE BAT LINING!!!"