How can I find reliable information on extending my NON-O retirement visa in Thailand?

to get the most recent and up to date information you can slog on out to your immigration office and get the hand out listing the requirements for that particular office (with over 80 offices they all don't do it the same way)

This is a good starting point BUT it is not the list to end all lists and you should always go get the hand out from your office BEFORE you go to get an extension so you can pull it off in one try.