Select a pillow-top mattress that comes with a box spring as the foundation.
Choose your desired size of mattress, such as king or queen, and a few foundation options for that size will be available with that mattress.
Decide between a full foundation, a slimmer foundation that will keep the mattress lower or a split box spring foundation, which is a box spring in two pieces useful for installing in areas with doorways or turns.
Choose a similar foundation when replacing an old foundation for an existing pillow top.