I think it might depend on your definition of "full". Out local JoAnn store does carry two types of boas, one being, I believe, turkey feathers. They are quite full (ie. "bushy") and about 6 feet in length. They also carry a thinner maribou boa in about the same length. The prices are actually quite reasonable for what you get.
If you are looking for something a bit more "upscale" you will need to consult a theatrical supply house. And of course, there are many. Here's where you can find most of them.
Go to
stage-directions.com
Click on
"resources"
then click on
"Stage Directions Theater Resources Directory 2003-2004"
then click on
"Costumes, Makeup, Wigs & Costume Supplies"
This will bring up a list of dozens of suppliers and their contact information. Some will also have a small blurb describing their services in a bit more detail.
Hope this helps!