http://anna-marie-bowman.hubpages.com/hub/Decorating-Your-House-For-Halloween is a unique web site with wonderful tips and suggestions on how you can use your Christmas sale items for Halloween.
Typically Christmas decorations go on sale as soon as Halloween is over. In some stores, Christmas decorations go up as soon as the school year begins. If they aren't out by September or October, they will definitely be up right after Halloween.
If you are willing to forever change your decorations, you could cut and paint them. The paint will help transform them from Christmas to Halloween decorations. You could also use fabric as a temporary solution.
A good place to try trading some of your Christmas decorations for Halloween decorations would be Craigs List. You may also want to consider donating them to a store like Savers. They will give you a store credit that you can then use to purchase Christmas decorations.
A store will usually decide its Christmas program once Halloween has passed. Some malls even being putting up Christmas decorations as soon as the Halloween decorations have been taken down.
I'd highly recommend checking Youtube on how to reuse some of your Halloween decorations for Christmas. They usually have some great ideas and would also be able to help you learn how to make your own to add your own personal touch.
The Christmas Tree shop offers Christmas decorations year-round at very reasonable prices. Seasonally, you can find some great lighted decorations at places like Target and Walmart.
There are websites available that offer decorations for Christmas and Halloween. I'd recommend looking on sites such as Amazon.com or Ebay.com for some really cost effective, tasteful items. They also have a really nice selection to choose from.
I like shopping at Kmart, Target, and Walmart for outdoor Christmas decorations. However, their stock may not be out yet. Check their online counterparts or Amazon for some ideas.
Halloween decorations are so much fun and so easy to create. You can buy some old wine bottles and decorate them with candles and black ribbon. Or burn new titles on old hard covered books that say things like "Book of Spells."
There are some places online that offer ideas on double-use ornaments. One site that I like is http://tlc.howstuffworks.com/family/halloween-decorations.htm.
Making your own Halloween decorations can be a lot of fun and it doesn't have to be expensive. Some great places to get ideas are http://iblue.hubpages.com/hub/Top-10-Halloween-Homemade-Decorations, http://familyfun.go.com/halloween/halloween-crafts/, and http://www.squidoo.com/halloween-homemade-decorations.