Not to be outdone by its televised cousin, Bongo Comics has been releasing a Treehouse of Horror issue of Simpsons Comics every year since 1995. Much like the show, each issue contains a number of short stories. However, the comic book showcases different artists and writers every issue, from all over the field. The artists are encouraged to adapt Matt Groening's style to fit their own (Sergio Aragones' take on Our Favorite Family was quite interesting), as well as throw in references to their own work (in a story drawn by Dan DeCarlo, the protagonists find themselves in a version of Archie's Riverdale).

The issues so far:

Some of the individual stories have been published in trade paperback format. Unfortunately, Bongo Comics has a rather random method of selecting which stories go into which issues, but they make up for it at least in part by including one- and two-page shorts, usually in the style of Groening's old Life In Hell "advertisements" or parodying Bil Keane's "Family Circus" style "follow-the-tracks-through-the-day".
  • Bart Simpson's Treehouse of Horror: Heebie-Jeebie Hullabaloo
  • Bart Simpson's Treehouse of Horror Spine Tingling Spooktacular
i can't find contents anywhere!

These trades can usually be found at local bookstore, as well as your local comic book store.


Sources:
http://www.informatik.uni-bremen.de/~tokra/simcomic/usindex.html
http://www.thefourthrail.com/reviews/snapjudgments/100801/bartsimpsonstreehouseofhorror7.shtml