The store with the cheapest yard decorations for Halloween would be Michaels or JoAnn Fabrics. They always run a coupon in the Sunday newspapers for 40% off a regular priced item.
Wal-Mart, Target, Walgreens, and Freds all carry inflatable Halloween yard decorations or varying sizes and prices, from small pumpkins to giant ghosts. Amazon also carries inflatable Halloween yard decorations.
For your best bargain, I recommend waiting until after the Halloween season. As I have found that holiday decorations are the best and cheapest to buy after the holiday without sacrificing quality.
My favorite place to find Halloween decorations for the yard online would be Amazon. They offer many options, as well as vendors. If you purchase enough, they offer free shipping.
You can get wreaths, yard decorations, fake spider webs and monster and ghost decorations in Wal-Mart and Walgreen's. You could also try Hobby Lobby, as they have many things that you could use for Halloween.
Yard decorations can be purchased at any local discount stores like walmart or Target. Dollar stores also often have these decorations but for a better price.
Target or Walmart usually have a great assortment of decorations for Halloween. You can get tombstones made from plastic or styrofoam to plant in the yard. There are battery powered hands to put in the candy bowl and a wide variety of decorations such as spider webs and spooky signs and window decorations to make it a great Halloween at your home.
A good starting point is http://www.partycheap.com/Cheap_Halloween_Decorations_s/31.htm but there are many others. Two of them are: http://www.frugal-save-wave.com/cheap-halloween-decorations.html and http://www.halloween-website.com/decorating.htm . A DIY site is: http://holidays.thefuntimesguide.com/2010/10/cheap_halloween_decorations.php . Enjoy your party!
Party City sell a wide variety of Halloween decorations,from inflatable to hard plastic ones at a cheap price as well. Their stores are usually located inside shopping plazas.
Although it may be difficult, Christmas decorations can be used to decorate the outside of your house for Halloween. An example of this would be white Christmas lights. These can be used to light up a collection of jackolanterns.
You can get some great ideas for scary Halloween decorations from a book called, "Extreme Halloween: The Ultimate Guide to Making Halloween Scary Again," by Tom Nardone. Inside the book are ideas for creatures to make and other ideas such as pumpkin carving.
There are some very simply Halloween decorations that require no thought whatsoever. Bowls with candy. Lights. Try just buying some hanging dummies and hang them from a tree in the front yard.