Why would you want to? A TEFL is to prepare you for the classroom - so a good course should cost money and it’s something we should invest in. Buying a course that you physically attend and have live observations are more respected my employers. Some employers don’t accept online TEFL certificates because they aren’t worth anything on a practical level.
Do education the proper way - don’t look for a free shortcut.
Remember, you want to be a teacher, an educator, a leader for your pupils, someone to look up to. Live it.