Hello there,
The padding you choose depends on what you are looking for and how much space you need to fill.
Typical premade helmet padding - Nice quality and coated in a fabric, but it may not be enough to secure your head if it is a heavily oversized helmet.
EVA foam: Good for support around the chin, but it is dense, uncomfortably stiff when used in a helmet, and will become heavy if used too much.
Upholstery (cushion) foam - Soft and squishy, probably the best choice for large and oversized helmets. It is itchy and may need to be lined with Lycra to avoid skin irritation.
Best of luck!