My palette of choice for mixing my watercolours is an old porcelain plate.
If I need a large amount of the same colour for a series of paintings such as for At The Pond, I will use porcelain bowls of different sizes.
I have plastic ones but I find they can get scratched over time and then are hard to clean.
The porcelain plate cleans up beautifully and the intensity of the white glaze gives the paint a transparency similar to the paper.
Just try to get pure white ones with no design or colour to conflict with your colours. The best and most inexpensive were picked up at an Asian food store near my studio.
Or just check out the nearest reuse store near you.