A computer Switch Mode Power Supply (SMPS). These can be found easily and modified or harvested for parts.
A SMPS ferrite transformer, before being cut in half.
Cross section of a ferrite SMPS transformer from a desktop computer. The many secondaries can be seen. Each layer has some foil to prevent EMI/RFI and other unwanted effects.
Inductors and transformers. I use many types of these components in my circuits. Some are ferrite, some steel, others powdered metal. I often color code them with paint.
Microwave oven transformers are a great option for the need power. They're cheap, plentiful and work right off 120 V or 220 V sockets. On the downside they're heavy, really heavy and get hot after a few minutes. This one is 5 or so pounds.
The secondary can often be cut off and one or more new windings wound in its place.