Sample sales are the best place to pick up a demo board. Picked up 3 of this years boards at last seasons burton sample sale for half price. The trouble is finding out when and where sample sales are happening.
You can usually find them listed somewhere online. Like the Burton reps out here just posted on FB that the sample sale is April 7/8. I know the sessions rep in the tahoe area already had his about a month ago. Ask around find out who the rep is in your area and go from there.