Paris Hilton, Fergie, Carmen Electra -- Showing As Much Skin As Possible for Halloween

If you're expecting Paris Hilton, Carmen Electra, Fergie, and Dolly Parton to dress up for Halloween in something slutty ... you're smart -- but TMZ knows exactly which super-revealing outfits they picked out this year.

A rep for L.A.'s famous Trashy Lingerie store tells TMZ, all four ladies stopped in to purchase various sexy costumes.

Paris picked up 4 costumes ('cause one is so bourgeois) for a total of $3,000 -- two different fairy outfits, a sexy Robin-looking outfit, and an Alice in Wonderland costume.

Dolly dropped more than 5 grand on a simple black leather pirate get up -- but it required custom tailoring for her, umm ... jack-o-lanterns. Hence the huge markup.

By the way ... a bunch of Hollywood costume shops told us what the most popular celeb-themed costume ideas are this year. Which one you got?