A friend got me this off of ebay. bike turn signal
It's not something I'd recommend spending a lot of money on--fortunately she won the bid for $1.99 or something. Spent more on shipping, probably.

The cords aren't long enough, and the controller doesn't have a way to mount it, like it's missing pieces or something. In general it seems a bit rickety. But, the horn is decently loud. It has 8 honks, 3 of which sound like a cell phone. I could have fun with that one--watch the motorists start searching for their phones. I got it mounted with creativity and cable ties.

Yesterday morning I got to HONK BACK at a car. Today I was disappointed; no one honked at me and I did not have an opportunity to honk back. Maybe tomorrow I'll have better luck!