Both methods are popular - the choice depends on your skills, budget and time. For making parts from scratch we have an informative DVD :
Scratch Building Basics
It requires a builder with special skills, knowledge, and an attitude complete with persistence and imagination. Not all of us are cut out for a project like this… although most of us can appreciate the rewards that will result!
Details of the fabrication process are explained and demonstrated - such as: how to translate blueprint plans to a form block drawing, cutting, sanding and finishing the form blocks using common tools; choice of form materials (plywood, MDF, particle board, hardwoods, etc); tools for cutting aluminum sheet, (power shears, hand shears, scoring knife), forming rib blanks around form blocks (how many hammers can be used!), fluting techniques for curved edges, cutting lightening holes, various methods for deburring and polishing, sheet metal bending using a home made brake, and much more.