I've seen a few of those trim pieces from SMB and they are usually just screwed in. Any screw should do, but sure you can find the trim cap style from SMB or anywhere really.
The crank mechanisms wear out. First try tightening the screw in the middle of the knob. If that doesn't do it, remove the know and see if the gear teeth are chewed up. Sometimes it's the obvious ones where the knob connects and sometimes it's the internal ones which requires a window pull and some know-how to fix. If you're near Tennessee I'll be glad to help but I see you probably aren't.