With Gene Autry's classic version of "Rudolph the Red-Nosed Reindeer" playing in the background, Sarah Stockton-Brown tried on perhaps the trendiest top of the early winter season: a black wool vest embroidered with red Christmas bows and white bedazzles.

Like many people in their mid-20s, she's going to an Ugly Christmas Sweater Party, and she's hoping to win some serious bragging rights.

"I'm looking for something really atrocious, something really offensive," said the 26-year-old while checking her reflection in the mirror at the Sazz Vintage pop-up shop in Old City, dedicated to nothing but ugly-yet-totally-awesome Christmas sweaters.

For years, what your Aunt Louise wore to a holiday party in 1982 would be considered anti-cool - think embroidered reindeer, fuzzy snowmen, sparkly candy canes. But as more people throw ugly sweater parties - with contests like Most Velvet or Creepiest Santa - a new tradition seems to have taken hold. Ugly sweaters are the next must-have holiday item, keeping vintage stores and online retailers scrambling to fill the demand. One designer is even marketing Hanukkah sweaters that are just as garish as their Christmas counterparts.

Amanda Saslow created the Sazz Vintage Ugly Christmas Sweater Store last November, and so far this season, she's sold hundreds of sweaters, sweatshirts, and vests in the store and online. She's even supplied sweaters to local news shows and Late Night With Jimmy Fallon, she said.

While some people want to look as cool as possible, partnering their sweaters with tight jeans or hip shoes, others go for an entire ensemble. So Sazz also sells holiday-themed turtlenecks, green and red dresses, and lots and lots of plaid.

Yet with such high demand for ugly Christmas sweaters, the supply at thrift stores has shrunk from abundant to slim in just a few years, said Saslow.

"People recognize it's a thing now, so they're not donating them as much," she said, noting that she's seeing fewer come her way in bulk vintage trading. "We buy them all year round and hoard them."

Plenty of websites have popped up dedicated to selling them, too. Uglychristmassweaterparty.com started out a few years ago as a blog about Christmas sweater trends but has grown into a business that's on track to sell 20,000 sweaters this season, said Brian Miller, 31, who runs the site from Crown Point, Ind. As of late November, there had been 114 orders shipped to Philadelphia.

"It doesn't matter if it's a holiday party at your office, a pub crawl, or a regular family party, this gives you the excuse to goof off and have fun," said Miller.

Last year, Philadelphia was host to an ugly-sweater pub crawl on South Street. Denver recently held an Ugly Sweater Run with free beer at the finish line, and the national initiative Stand Up to Cancer created a campaign to have people pledge to wear an ugly sweater for two weeks while raising money for cancer research. Fishtown bakery Whipped Bakeshop is even offering boxed sets of ugly-sweater cookies.

William Aultman, 22, attends an annual horrible-sweater jam with a group of friends from the University of Pennsylvania, serving as a playful reminder of his youth.

"My parents always bought me and my sister ugly sweaters every year," he said, noting that his parents didn't consider them heinous. "They managed to find the same ugly sweater in different sizes."

This year, Aultman plans to go all-out - he'll wear plaid pants and a turtleneck to match his blue and white snow-landscape cardigan with embroidered Christmas trees and snowmen.

At the end of the night, the group of eight to 10 friends plan to snap a "family photo" of everyone looking oh-so-dorky, yet oh-so-cool.

Every winter, Evan Mendelsohn seemed to find himself combing through the women's section at Walmart or a vintage store to find something to wear to an ugly sweater party. So last year, he started TipsyElves.com, which sells new Christmas sweaters - albeit crude ones. One depicts Santa Claus peeing the words "Merry Christmas" into the snow while another shows a silhouette of three reindeer in compromising positions.

"For a lot of people, these ugly Christmas sweater parties are a second Halloween where it's socially acceptable to dress up regardless of your age," said Mendelsohn, 28, who quit his job as an attorney to focus on TipsyElves and a few other niche online retail ventures, headquartered in San Diego. "Even older generations are starting to get the joke."

Carin Agiman, who's Jewish, never felt right wearing Christmas stuff to ugly sweater parties, so she founded Geltfiend.com, a site selling ugly sweaters of the Hanukkah kind - picture a red-and-white cardigan with embroidered dreidels, Hebrew letters, and wooden buttons. Another has large yellow and brown polka dots with menorahs inside.

If Adam Sandler can get Hanukkah songs played on the radio and Hanukkah movies made in Hollywood (remember Eight Crazy Nights?), then why couldn't a Hanukkah sweater company work?

"Hanukkah is full of chocolate, gambling, and oil," said Agiman, 27. "If we can't figure out how to market it, people are just going to rush though Hanukkah to get to the Christmas parties."

She says the sweaters she designs (and retails for $65 to $70) are the perfect blend of irony and sincerity.

"I felt like I needed to represent in some way," she said. "I love going [to ugly sweater parties] but I wanted to go as myself. I never wore a Christmas sweater, but a lot of Jews have out of necessity."

So far the reaction from customers has been "super positive," and thankfully, she said, even the most religious Jews seem to understand the humor. That's because she's not trying to meld the holidays together, only to give Jews something ironic and comfy to wear.

The company started in August with donations from the business-launching site kickstarter.com and has sold more than 1,000 sweaters so far. "I wasn't the only Jew feeling left out around the holidays by people wearing fancy Christmas gear," said Agiman.

She's been wearing her sweaters around Berkeley, Calif., since early November, and she's been getting some interesting looks.

"Some don't realize it's Hanukkah [themed] at first glance," she said. "Watching people do a double take is pretty funny."