Where to find error_log file(s) depends on the host and configuration.
You may start within the 'wp-admin' folder, and then the next level up.
Also, you might find them on the root of your webhost in files like "error_log" or "php_error_log". Or maybe in a logs folder beside your public html folder.
Failing that you'll need to check with your host...
This blog article may help: