In many rural areas, women make their own hair extensions out of horsehair that is usually taken from the horse's mane or tail.
The hair is cheap and can be easily sewn together to form an extension. The horsehair is soft, and can be dyed to any color. And the texture, or consistency, of horsehair compared to human hair is close enough that nobody would notice.
Another possibility is to visit a thrift store. There they have wigs and other attachments that can be easily modified into a hair extension.
Sill another source of cheap hair extensions is your local beautician. The best candidate is to find a customer who wants the Britney Spears or Lindsey Lohan look -- i.e., bald -- and pick up the clippings.