If during TIA you had difficulty speaking, where might the damage be?

Location. Left-sided strokes and tias tend to cause speech problems since speech centers are located in the left brain in most of right-handed people.
Left temporal. The speech center is in the left temporal region in an area named broca's area. This area is vascularized by the left middle cerebral artery. So likely, the blockage is in that artery, downstream in or around broca's area, or upstream in the internal carotid which feeds the mca. If just speech is affected, it's likely downstream in a branch of the mca.