S4 Failing Headlamps

[tintinmt]

Hi, my wife's S4 (B7) Cabrio has developed a headlamp fault. They seem to go on and off occasionally. Usually just one side goes off and on again. Sometimes the whole lot seem to go out.

I have trawled the various past posts but still have questions.

I have recovered from realising that the bumper really needs to come off and can do this.

What should my plan be?

Do I need to get bulbs?, igniters? ballasts?

Where should I source the parts?

If I just replace the bulbs and that fails to cure, do the igniters and ballasts require the bumper off again?

Would I be better off just replacing the whole lot in one go? Cheers.