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.
You can try hgtv.com as they offer many different venues for buying Halloween decorations and show you different ideas from real houses and show houses that you can use to incorporate into your house.
You should begin at your local hobby or craft store. There are many great supplies and ideas in these places. You can also do a search for this topic.
A good place to find do-it-yourself ideas for decorating an entryway for Halloween would be www.doityourself.com or at a local craft store.
I know of two really good places to find ideas for decorating Halloween treat bags. You can find them at Party City. I'm not sure if you have one near you but I'm sure they have a website. Also, Oriental trading Company. They are a catalog company with great ideas.
If you are looking for unique costume ideas, you can find them at http://www.essortment.com/halloween-costume-ideas-50-unique-weird-ideas-52253.html. You can also find unique decorating ideas at http://www.halloween-website.com/decorating.htm
I like to use the Better Homes and Gardens website for my decorating ideas ( http://www.bhg.com/halloween/outdoor-decorations/ ). The great thing about this website is that it doesn't just give you Halloween ideas, but it has ideas for every holiday!
You can visit Youtube to find various ideasfor your Halloween party. You can also visit Michaels art store for a large selection of Halloween ideas, themes and supplies.
Martha Stewart (http://www.marthastewart.com/photogallery/halloween-decorating) has a plethora of tips and tricks for Hallowe'en decorating. Better Homes & Gardens also has a large selection of ideas, though they focus more on outdoor decor (www.bhg.com/halloween/outdoor-decorations/).
You can find decorations for your door online at this site: http://holidays.thefuntimesguide.com/2009/09/halloween_decorating_ideas.php. It has lots of ideas, as well as some videos of ideas, not just for decorating the door, but for decorating other areas of the house.
You can find some awesome patterns for decorating a pumpkin at http://www.marthastewart.com/photogallery/pumpkins. You can also find more ideas at http://www.countryliving.com/crafts/projects/pumpkin-decorating-1009
There are many great websites to find affordable ideas for Halloween costumes! Here are a few links: www.robinsfyi.com/holidays/halloween/costumes.html and familycrafts.about.com/cs/halloweentheme/a/091399.html.
Party city offers great unique halloween decoration ideas. They always have up to date costumes and party supplies for every occasion. They also have catalogs to help with decorating ideas.
Home made halloween decorations are very affordable and creative. I like to go to oriental trading to find ideas and supplies for this type of decorating.
There are many great websites that offer great Halloween decorating ideas. Some of my favorites are BHG.com, marthastewart.com and hgtv.com, they all offer ideas no matter what your budget is.