Try:

http://www.fiberglasssupply.com/

It costs more if it says Aircraft

To you three guys with photos of broken hulls, how did you do it? I'm in the market for an 18 and your experience has been helpful.

What ever you add to one of these boats needs to be light! Don't fill it up with heavy basketballs or pool toys! Expanding foanm is a no-no because it has a tendency to absorb every drop of water that enters the hull, making the boat heavier and heavier.
Get a strong plastic bag, fill it with styrofoam balls, seal it up and then squeeze it through the inspection port and back past the centre case. Repeat the exercise till you are satisfied you have the reserve buoyancy you require.

Skipshot,
The problem with those hulls occurred mid 80's. Keep in mind while looking for a boat you don't have to avoid a boat of those years. Many of those boats received upgraded replacement hulls. There might be a way to tell if they've been warrantied by looking at the VIN. # on the back of the hulls. Maybe Matt might be able to tell you how to find that info out. When looking at boats check under the shear by the front beam. If you find cracks in the gel coat that extend more than a couple of inches be careful. If they're 10 inches long run away. I had a 81 that I bought in 85 and there were some cracks 2-3 inches when I got it and after 17 years of heavy use they never got any bigger. Good boats.

I saw anchor plates on several of the busted TheMightyHobie18 hulls. Excess load from wings, and lightened boats under design weight....may = trouble.
At a Nationals in Florida, Larry Mondragon was putting his brand new TheMightyHobie18 into the water- weight only 408 pounds! (YAYAYAY- a real winner!) HULL BROKE ON THE BEACH; never even made it to the water!