This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
Wholesale distributors of lumber products. Home Improvement Centers,
such as Home Depot or Home Base, and Lumber Yards may or may not stock
MDF products, but they will be able to order it for you. Contact
woodworking or cabinet making shops in your area, ask them where to get
it or if it may be possible to buy the material from their business.