i got a twin pack at Halfords....not blue, but 50% brighter, about £20 iirc, didnt seem like extra 50% but a definite improvement, and good enough for me...they do BOGOF offers now and then which is better
I used to use Osram Nightbreaker Plus which offered a marked improvement however when changing to HID recently I noticed some scorching of reflector. Whether this would have happened anyway (took about two years) i don't know.
Why not go the whole hog and go HID? Now they even have coated bulbs to give a legal beam pattern...
Best place is probably on-line. Look for Osram as they are of decent quallity and used by BMW anyway. If you get one in BMW box, it will be 3x the price you can find on-line.