What can replace the reflector for photography? Like foam board, cardboard or something?

It is generally not recommended to use foam board+tin foil to reflect light, and so is cardboard.

Because such a DIY reflector often has shortcomings and gaps in color temperature and brightness, it needs special attention when shooting. Whether it is used or not is often limited by the specific sunshine environment. If you make your own reflector, it is easy to have a big jump in temperature difference and uneven brightness dispersion. The color temperature is easy to handle, and can be adjusted by the later Raw, but the brightness cannot be adjusted, so it is not recommended.

At present, the price of three-in-one or five-color reflector is within 100, which is not very expensive. If there are many portraits, I recommend one. The benefits of reflector are absolutely beyond your imagination!