Translation:If they like it, I'll buy them some tomorrow.
In English, you can have "some" of a singular noun if the noun is uncountable. In this usage, "some" refers to an unspecified quantity. For example, consider "I need milk; I'm going to buy some tomorrow." "Milk" is a singular uncountable noun, and "some" refers to an unspecified quantity of milk.
When the noun is countable and singular, then "some" usually is not appropriate to use. For example, something like "I want an apple; I'm going to buy some tomorrow" is strictly speaking not correct, even though it might be said colloquially. It would have to be either "I want an apple; I'm going to buy one tomorrow" or "I want apples; I'm going to buy some tomorrow."
Everything needs a first time. "En" represents a noun previously mentioned. In this sentence the referent is not a noun but the pronoun "ça", itself representing a noun. If you develop this sentence with more context you get this:
- S'ils aiment [le pain/la soupe/les croissants], je leur achèterai [du pain/de la soupe/des croissants] demain.