I purchased some at a LQS that was going out of business! Felted wool is soooo expensive!
The best "inexpensive" source I have been able to find is as mentioned above...
Salvation Army, Good Will and used clothing/resale shops.
The best score ever was a wool blanket - so awesome because the amount of fabric without any seams!!
Look for boiled wool jackets or other wool coats without too many seams and take them home and boil them yourself!