Many of the scratch builders use bike helmets or hard hats as the basic shape. You could get lucky at your local thrift store or garage sales and pick up a used helmet for a couple of bucks.
Other than that, there are pepakura files out there, that you can cut and glue. Then bondo to fill in the gaps and sand your life away on the dome.
A quick search on the forums for "scratch build bucket" should give you a few more WIP's to look at.