There's no need for anything fancy when it comes to sidelights, as Scooba_Steve says above you just need a W5W fitting. Something like these
http://www.ebay.co.uk/itm/2x-BRIGHT...864186?hash=item237b80587a:g:g7EAAOSw2xRYeJFx would probably get the effect you're after for next to nothing without needing resistors or any adjustments.
I went through a bit of an LED obsessive phase and through trial and error I replaced every bulb inside and out with the brightest and whitest bulb for each available fitting without causing any bulb errors. The only one I was never able to find is a H21W that doesnt cause an error for the rear fogs. Other than that you wouldn't believe the difference a crisp bright LED can make to the overall asthetics, indicators et all.