Harry bought strawberry-and-peanut-butter ice cream for himself, Ron, and Hermione while they strolled around Diagon Alley during their visit in 1992 (CS4).
When Harry spent three weeks staying at the Leaky Cauldron and hanging around Diagon Alley in August of 1993, Florean Fortescue served Harry free ice cream sundaes and offered advice about medieval witchcraft for a report he was writing as summer homework (PA4).
Hagrid bought ice cream for Harry on their first visit. Although it doesn't specifically state that he purchased it at Florean Fortesque's, it seems likely. He bought Harry chocolate and raspberry with chopped nuts (PS5).
Q: I have to ask this little thing that's been on my mind... what happened to Florean Fortescue?
JKR: Uh, he was killed. I didn't want that to happen. Bizarrely, my best friend, after I named Florean Fortescue, she went and met, and is soon to marry, a guy called Florean, which is very bizarre, because it isn't exactly a common name, is it? Because of him, I was very attached to Florean Fortescue but no-- Yeah, he died. He died.
Q: He was just an ice cream man, wasn't he?
JKR: The Scottish Book will reveal there was more to that than subsequently made it into the books. It was one of those little sub-plots that had to be sacrificed because it was not really leading anywhere, but I did have a sub-plot planned for Florean. And it was to do with the Elder Wand, so I will definitely put that into the Encyclopedia. (PC/JKR2)
On Pottermore, Flourish and Blotts was on the left side, next to Madam Malkins (Pm).