Broken Channel
Thanks for writing in! The white plastic you are referring is what is referred to as a jamb liner. Given the significant amount of damage to it, I do suggest having that replaced before replacing your channel balances. Unfortunately, we would not be able to supply you with the replacement for it, so it is suggested to contact the window manufacturer or a supplier that might carry their replacement jamb liners.
In regard to your balances, I am confident that we can supply you with the replacements for them. Although, I'll need to see a photo of them removed from the window to confirm. However, I might reiterate that it would be best to repair the jamb liner prior to replacing your balances.
Please let me know if you have any questions.