There was actually a show on Spike TV (Musclecar) where they created their own fender flares for a mustang they were going to track. They used the Blue foam from wherever (floral store or home depot), and stated that you can use anything to create the shape you want; i.e. any type of foam, sheet metal, cardboard, WHATEVER gets you the shape you are looking for.
The only thing you need to worry about is a release agent. They actually used aluminum foil as their "release agent." They got the flare to a shape they liked, wrapped the foam in aluminum foil, then started laying fiberglass over it. They had stated they did 3 layers of fiberglass for strength.
(Oh, and on a side note, they were testing different glues out to try and find something that would allow the blue foam to stick to itself so they could get some depth to the flares. They tried, I believe, 5 or 6 different glues out and finally found one that would actually hold the foam. They didn't 'tell' the viewers what glue they ended up with, but I'm 99% sure they found that the "Gorrilla Glue" ended up working the best. You can get this stuff at Kragen Autoparts or wherever.)
I have never fiberglassed before but it honestly didn't look too hard. Set the foam up on your fender then start sanding away to shape it into something you are looking for. Once they had their mold glassed, thats when they started to sand, fill with kitty hair fiberglass, sand more, then bondo, sand, bondo more, sand more, etc. I'm sure it is a very tedious process of sanding and filling, but the end result will be worth it if you just take your time.
Also, one last side note (I swear), the guys creating the flares also molded in bolts to their flare so all you need to do is drill a hole in the fender, and throw a washer and nut on the other end. Looks clean and took no time to do.