They make turn signal blinker capsules (the little silver cylindrical thingy with the electrical prongs on one end) that have a buzzer in them! It buzzes with each blink of the lights. Sorry, I can't recall what 'A' supplier I got it from - it may have even been a local auto parts store. Check around and call the 'A' suppliers. You'll find it.
I personally ended up not liking the buzzer type so much because the buzzer was so loud! I had to wrap a piece of duct tape around it to muffle it. ALSO - the buzzer drew enough current that when the engine was at a low idle, the signal lights wouldn't blink - not enough juice to drive the bloomin' buzzer!. they just stayed on. AND, at higher engine RPM's, the headlights dimmed up and down when the turn signals were blinking! Not cool. It may have been a "weak" or mis-adjusted generator but I just yanked the module and went to an ordinary silent type. No problems now - works great.
Without the "reminder buzzer", I just had to discipline myself to turn the signals OFF after making my turn. But I still ain't perfect. I don't know what's more embarrasing - people looking at me at stop lights because of the funny, loud blinker-buzzer - OR, now, people pulling beside me to tell me I left my turn signal on.
Earle