Answer:
There is no restriction on purchases on a fast day, with the exception of Tisha b'Av, even if the object has no use for a Mitzva. A fast is neither a holiday when transactions are forbidden nor mourning when one must refrain from purchases.
However, it is certainly praiseworthy to avoid actions that will distract one from the seriousness of the day. So, while should refrain from shopping for frivolous items, there is no detraction from the spirit of the day in pursuing spiritual acquisitions.
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