when I was in the market for a 156 I spoke to a local garage that I was using for most servicing and MOT and they said they once had one in for the side light bulb and had to take the whole front bumper off to replace it. Dunno how true this is, they are just a independent but they always looked after my other cars really well.
Sorry if this is completely wrong and scares you half to death.
as above but beware your original holders may crack when you push them back but as you are replacing them it dosen't matter, just make sure the paintwork is clean first so you don't damge it (salt is very abrasive)
The orange bulbs are sold by the "Big H" and other motor factors. They're the bulbs without a metal socket.. just a thin piece of wire each side of a moulded glass base. You'll recognise them when you see them.
Do the ones on Ebay fit perfectly Ive heard some stories of some not being exactly the same shape!! For the price Ebay sellers want is it safer to get some later model 156 clear repeaters direct from Alfa (under £20 a set with bulbs), id imagine they would be an exact match for the old ones. Its the 1st job on my list as i dont like orange repeaters on a dark car.