Should you want to buy a small quantity of blank, pre-constructed game boards, Bare Books has a couple of really basic quad-fold board. If you buy a blank game board, you'll still want to read the glossary entry on quad-fold boards, and follow the tutorial links to figure out how you need to print your board on your home printer. You can also print your game board at any print shop with a wide format printer, but realize that they'll charge you a fortune.
Board Game Design has blank boards and boxes, but they are quite expensive, and are not suitable for production, due to the prices. Maybe some will find a value for these for prototyping.
If you want a lot of blank or printed game boards, look at the article on Board Game Manufacturers for a list of possible people to handle your needs for the right price.