Where can one find the 'disputation of Barcelona' between Pablo Cristiani and the Ramban?
The best (English language) source, in my opinion, is Hyam Maccoby's Judaism on Trial: Jewish-Christian Disputations in the Middle Ages (Littman Library of Jewish Civilization, 1982; later adapted as a play viewable here). In it you will find a translation of the Ramban's Vikuach, and of the official church account of that disputation, together with an extensive introduction and commentary. The Ramban's Vikuach is also translated by R' Dr Charles B. Chavel, and published as The Disputation at Barcelona (Shilo Publishing House, 1983).
The original Hebrew text is found in Kitvei haRamban I, pp302-320 (ed. Steinschneider, 1860).
It's in the Kisvei HaRamban. Volume 1, I think.