The easist solution is to buy the full thing from Pasco, but at $319 it is expensive. A kit that includes everything but the board and air supply is cheaper at $139 but is still costly but not outrageous, especially if you already own a cordless leaf blower. I really recommend this kit.
Building your own is pretty cool too. This set of plans from Bill Beatty (my hero) explains what to do really well and will set you back less than $50 plus the cordless leaf blower.
However, you can combine both of these sets to get a great design for a bit less money. Make the skirt in Bill's plans by using ballistic nylon like Pasco uses. Buy a yard and a half of 330 or 500 denier cordura nylon, like this stuff. Cut it into a circle a couple of inches larger than your hovercraft disk. Hem a hollow edge all the way around the outside of the circle. Follow Bill's advice for cutting circles out of the center. Thread a piece of stiff wire around the seam. By pulling the wire tight, you can keep the skirt attached to the board.
You will need to seal the skirt along the edge. Staples and silicone sealant will probably work, but heavy staples and heavy tape work well too. A rubber bumper like the one Pasco sells used to be available online, but the company where I saw it has gone out of business. (Thanks Steve for telling me!) I'm looking for a replacement.