The opening and closing scenes of "White Christmas" feature World War II soldiers and veterans. From privates to generals, dressing as a military officer from the 1940s U.S. Army is a significant choice from the movie's costumes. You can find a costume to rent or purchase or try putting together your own facsimile by acquiring a khaki shirt and pair of pants from a uniform supply store. Accessories such as a hat, belt and medals can add to the effect. If you're a woman, you can replicate the snappy female military outfits worn by the Haynes sisters during the final performance.
Some of the most recognizable costumes from the movie "White Christmas" are the ones worn by the stars Rosemary Clooney and Vera Ellen. Why not create a copy of one the blue dresses and lavish fans of their "Sisters" costume? Even men can go drag and wear this one, as when Crosby and Kaye recreated the number. Or find a vintage 1950's full skirt and create one of the sisters' more casual looks. The ladies' fur-trimmed winter coats would also make eye-catching costume choices. You could also recreate the busybody housekeeper's look (dowdy sweater, skirt and glasses, perhaps with an old-fashioned telephone prop like the one she used for her eavesdropping) or the classic bobbysoxer teen look of the General's granddaughter Susan (jeans, flannel shirt and penny loafers).
A major portion of this movie is dedicated to rehearsal and performance of Wallace and Davis' show to help their old General make a go of his Vermont ski lodge. You will see a variety of vintage Broadway-style dance wear -- from the rehearsal clothes the gypsies (a Broadway term for ensemble singer-dancers) wear to the more fanciful costumes of the big production numbers. Why not dress up as a modern dancer a la the Martha Graham-style dancers in the "Choreography" number? Or go for the more uptown look of the "Mandy" number. Dance recital supply stores provide costumes that could be amended to create the proper effect.
The big finale of the movie features the stars (plus children) dressed up in classic red and white Christmas attire. The men sport basic Santa suits -- white fur-trimmed jackets and Santa hats, big, black belts and red bags slung over their shoulders. The women are breathtaking in full-skirted Mrs. Claus outfits that mirror the men's looks. There are also boys dressed as miniature Santas and girls dressed as dainty Christmas ballerinas. Any of these costume ideas could be recreated from readily available Christmas costume offerings.