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.
The first publication of the Vikuah is found in Tela Ignea Satanae (Altdorf, 1681), a book with polemics edited by a xtian hebraist named Johann Christoph Wagenseil followed by a latin translation of the dispute.
Its text has interpolated expressions by jewish copyists, however this gives a sense of what occurred in the jewish mentality given the missionary efforts in those times.
See the edition here in pages 24 ff.