Did John Calvin ever read Thomas Aquinas's Summa Theologica? I've read in a few places online (here, for example), that John Calvin did not have access to Thomas Aquinas's great treatise, Summa Theologica.

This is mind-blowing.  Consider: The last edition of John Calvin's Institutes was published in 1560, four years before his death.  Thomas Aquinas stopped working on Summa Theologica when he died in 1274, almost 300 years prior.  Thomas Aquinas died in Italy, while John Calvin was born in France and died in Switzerland: two countries that share a border with Italy.  How could it take 300 years for a seminal work like this to travel a few hundred miles?

I'm not sure that we can easily attribute this to negligence on the part of Calvin, as he clearly valued interacting with (and often criticizing) his intellectual forebears, as seen in his frequent references to church fathers and the Scholastics (the "schoolmen").

What is the evidence that Calvin read or did not read Summa Theologica? And if he never read it, what prevented him from doing so?
A smoking gun is in the references that John Calvin makes to Thomas Aquinas in his own book, The Institutes of the Christian Religion (II.11.4 and III.22.9).  This is evidence that Calvin at least knew of Aquinas, which suggests that Aquinas' most important work had reached France or Switzerland and that he would probably have read it.

Mark J. Larson says in Calvin's Doctrine of the State, page 27, that Calvin read Aquinas either directly or through intermediate sources (citing Lane and Wendel as his own sources). Larson says that although Calvin did not explicitly connect his teaching on the just war with Aquinas, it could well be the case that he had read his treatment De Bello in Summa Theologica.