Black Friday is still three weeks away, but you'll find lots of Black Friday sites already on the Web. They post ads that can help you get ready for the big shopping day after Thanksgiving.

But the founder of ConsumerWorld.org, Edgar Dworsky, says you need to be careful because sometimes ads are posted prematurely.

"These are just kind of the pre-production ads and items change, prices change," he said. "So what you see may not be what you get on Black Friday all the time."

You know the information is accurate when it's on the retailer's site. Dworsky says Lowe's has already posted its Black Friday ads on a site called ShopLocal.com.

Of course, you can't buy these products until Black Friday. So here's what you do:

"A couple of days before Thanksgiving, go back and check to see what the ads are there and you can pretty well rest assured that by that time these are the final versions and you can do your planning."

Edgar says his two favorite Black Friday Web sites are BlackFriday.GottaDeal.com and BFADs.net.

Right now they already have Black Friday ads posted for Sears, KMart, Lowe's and Old Navy.

PC World recently named DealNews.com as one of the best Black Friday sites on the Web.