You can find personalized Christmas decorations at several websites. Squiddo.com has a variety of Halloween themed Christmas decorations for you "dark tree". Or, you can try searching for ideas to create your own homemade decorations.
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.
A good place to find fancy printable Christmas decorations during the Halloween season would be the internet. Many sites offer holiday themed ideas all year round.
Visit www.HGTV.com. They really have a lot of good and clever ideas for holiday home decorations. They can also provide you with some fun ideas for making decorations for halloween as well.
There are many sites to give you cheap ideas. Some of them are www.halloween-website.com/decorating.html and www.bhg.com/halloween/outdoor-decorations/
Scared-out-of-your-wits.com is a good place to look for these types of ideas. It has dozens of ideas to keep your Halloween holiday classic, yet still scary.
You can find alot of good ideas for arts and crafts online. A good site for easy Halloween Decorations is http://familyfun.go.com/halloween/halloween-crafts/ Good luck with your students hope you can find something that you and the children will enjoy.
Oftentimes Christmas decorations will be on sale before the "main" Christmas season so you may find good deals at Halloween time. Usually peak time is when they will be most expensive.
Visit Martha Stewart's website which is MarthaStewart.com. You should also go to hgtv.com as well. Both site will give you great ideas for Halloween decorations.
I think the best way to do this is to do a Nightmare Before Christmas theme (ideas here: http://dapolonia.com/46-halloween-glass-ornaments) or to have a Christmas tree with Halloween decorations hanging from it, such as bats or pumpkins.
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.
It depends on what costumes and decorations you would like to create. You can find some ideas for costumes here: http://costumeideazone.com/. As for decorations, the following site has a lot of great ideas: http://tlc.howstuffworks.com/family/halloween-decorations.htm.