Yes u can take re entry permit at the airport as well before going to immigration counter. This is very easy and take few minutes. Single re entry is 1200 and multiple reentry is 4000. If you have time, you can go to chaeng wattana also few day before leaving thailand
Yes you should have taken the re entry permit at the airport before passing immigration. You could have taken at some immigration before deciding to leave. Now u have to apply new cisa