How does one parenthetically amend a noun in a sentence so that both the singular and plural form are included?

In your case, you can just use the plural:

where their favorite items were purchased

since this doesn't necessarily require that each woman have multiple favorite items. (Consider "their hearts", which doesn't require that each person have multiple hearts.)

If it's necessary, for whatever reason, to emphasize that some women might have specified only one favorite item, then you can write out both the singular and plural versions of the subject, conjoined with "or", and use the verb that agrees with the plural version:

where their favorite item or items were purchased

(By the way, I'm guessing you already realize this, but: your sentence is not a complete sentence. It seems to be just an adverbial.)