For large enough quantities of paint, you can buy empty tubes and put the paint in there. Any good sealing plastic food storage bin (Lock & Lock) or glass jar would work too.

For smaller amounts, Paint Savers work really well to keep the paint good for at least a week. Paint Savers PS7 Paint Covers https://www.amazon.com/dp/B008UGB4T2/ref=cm_sw_r_cp_apa_i_vV-mDb8SADZVJ

They have (had?) their own official website too where you can order, but I can't seem to find it.

You can also get a palette that seals air tight. I have this neutral grey plastic one:

Mijello TruColor Neutral Grey Palette - Artelier Airtight 9.5x13.25 in https://www.amazon.com/dp/B005G3ZRAY/ref=cm_sw_r_cp_apa_i_TZ-mDbPXP1GJ3