howdy! active medibang user for a few years now
if you're looking for more diverse speech bubbles you'll want to set up a medibang cloud account (the sign in that usually appears when you first open the program) afterwards you can track them down in the materials section, it should be the icon that looks like a speech bubble on desktop iirc
there's also more that you can download and it sometimes helps to check their site (either the main medibang site or artstreet) to figure some things out or locate more hidden assets