It is preferable to acquire all products for Pesach, prior to Pesach.
The ruling of the Mishneh Berurah - that Ashkenazim who may not eat Kitniyot, rice etc. during Pesach, may nevertheless consume foods containing Kitniyot, provided that A) Kitniyot is less than 50% of the product B) Kitniyot are not visually identifiable C) these foods were not specifically made for those who keep the customary ban of not eating Kitniyot. ...... This permits all lemonades, which may use corn syrup, and chocolates which may use lecithin.
Liquid or creme fillings in chocolates might well contain Chamets and require KLP certification.
Carbon dioxide gas sourced from Chamets or Kitniyos may be consumed during Pesach.