If you want them to be very bright, you need to get those xenon gas bulbs like the hyperwhite overwatt headlight bulbs, but in a regular twin filament bulb.
The only ones I have seen are Not made in the U.S., and are not cheap either.
I believe they don't come in many different sizes etc. I have seen them at pep boys in the 1157 type which can work in the indicators of your car. I have used 1157 bulbs before in mine.
I recently just switched to full european, proper bulbs for my front and rear in clear. The fronts look dimmer than the headlights when off, but when both filaments are lit during the flashing, they get much brighter.
Justa thought, why not check some of the local rice rocket shops. they probably have them, or tey jcwhitney's site.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.