Hi cshapiro.
I want to apologize for being taken as obnoxious. I never intended to be rude, and I hope you will forgive me.
Based on my own ignorance, I had no clue there is such a thing to prefer in a shirt something like square buttons. I truly did not know.
Often times there are posts here meant to be tongue-in-cheek, and it is not easy to determine in 100% of the cases.
In any event, I apologize.
Secondly, I cannot answer your question and I dont even think I have seen dress shirts with square buttons, or maybe would not have noticed.
I tried amazon.com and all I found were “Harvard Square” brand shirts and shirts with square hems or square cuffs.
I would imagine a good suit shop can give you an immediate and accurate reply on obtaining such shirts.