This is nothing more than a standard dynamo front bulb - 6V 2.4W is what you really need (the rear is 6V 0.6W). Most Northern European bike shops carry screw-in Halogen versions of them for 'Union' brand headlamps of your vintage:
If you are not running the rear light from the dynamo then you need a 6V 3.0W bulb in the front. (2.4W + 0.6W = 3.0W).
The problem with the simple two-bulbs-in-parallel circuit is that if you remove one light (e.g. a broken wire) then too much current goes through the remaining bulb and it blows.
Think about it, go to the bike shop when you have a problem with the bike, not the electronics store or the car-parts store or the aromatherapy store. The staff in the bike shop do know a thing or two, they have heard of bicycles running dynamos and, even if they don't have spare dynamo bulbs to hand, they do have a supplier that they can get them from for you in a reasonable period of time for a price that will not cover their time dealing with your enquiry.
I suggest you visit the LBS first, they may only be able to get you the 2.4W version, if they can get you the 3.0W version then get it and a flashing light for the back. In that way you will have a rear light that works when you stop, more light up front and less likelihood of blowing the bulb (because there is no rear bulb to blow).
Alternatively, get a new front headlamp unit for the dynamo, these may be special order from the bike shop, however, such a unit will have niceties such as a better reflector and lighter construction.
