If you can access your database, finding comments is very easy. All the questions, answers, and comments are stored in table 'posts'. The column 'type' has 3 values: C, A, Q, where C is for comments, A is for answers, and Q is for questions.
Log into your database from the command line or phpMyAdmin and run the following SQL to find all comments:
SELECT * FROM `posts` WHERE type = 'C'