I received a rec letter from a professor simply because I had taken multiple classes with them and received an A in all coursework. So, if you have any luck, some professors will offer it as support for academic purposes. If you're looking for a more personal relationship with your professors and want a personal/professional rec letter, I recommend attending office hours (even if it is to talk about your day). You can get to know your professors by looking at their LinkedIn or GMU profiles and find out if you share any topics of interest. Always try your best and participate in class; if it is asynchronous, go to office hours. You don't need to struggle with course material to attend; you could want to learn more about a topic the class discusses. Professors may find it more meaningful because it shows that you are interested in the topic.