Hella Supertones. Can be bought for roughly $40-$70 a pair from nearly anyone that sells vehicle parts and can be installed in less than 1/2 hour. Had them in my old WRX and they definitely allow you to be HEARD/SEEN for a little guy on the road.
I tried a few months ago to install some in my VX and gave up for the time being. Install locations are limited since our grille is so inner concave (installing as seen like above that is). If you don't mind locating the horns elsewhere, you should have no issues at all.
I plan to pick up another pair and install some in my new STi and hopefully get around to going ahead and just mounting them in my VX.