I don't see how your question relates to suppressed technologies, unless you are attempting to replicate a suppressed device. The answer, though, depends upon precisely what you are trying to build. Ferromagnetic cores are made from ferrous metals, which are either iron or iron containing alloys. You can form a core from a single piece of metal, or you can fabricate a core from several pieces of metal that are laminated (or otherwise bound) together tightly. For example, most experimenters use bunched pieces of welding rod for Bedini SG coil cores. If you are building something that requires a large core, or one that must be machined to a specific shape (to match the curvature of a rotor, for example) you could start by looking at your nearest scrap metal yard. You will find a lot of cast iron and steel there (don't select stainless steel, as it has very little ferrous metal in the alloy). Normally you don't want to select metal that has already become magnetized, so take a small piece of non-magnetized ferrous metal with you and make sure it isn't attracted to the piece you are considering. Also take a small magnet with you to test the metal for attraction. You don't want core material that does not attract a magnet. If you don't have the equipment needed to cut and machine the core, you can take the material to a machine shop, along with a drawing and specifications. Alternately, you could ask the machine shop to supply new raw material for the core to be machined, or purchase the material directly from a metals supplier such as Metals DepotŪ - Buy Small Quantity Metal Online! Steel, Aluminum, Stainless, Brass
You can find many more supply companies if you do a simple Google search. If you need more help, try searching for a forum thread that applies to the type of device you want to build, and please try to be more specific when asking questions. We have many remarkable, intelligent, and experienced people participating in the energetic forums who would gladly offer their assistance, but first we must know some specifics about your project.
Hope this helps somewhat. Best wishes to you, Rickoff