i dont have those, but i have some similar ones. i got mine at a bicycle shop. they had some other ones that looked exactly like those. they mounted with a short straight arm like those, or with a 90 degree arm. they are only about $15-$20 dollars. mine work well and dont move or vibrate. i know a guy who has the other style also. he said his work just as well.
maybe check your local shop. cheaper, and you'll get them right away.