Are they standard (based on steerer tube width) and available from any decent LBS or do I need to source one specifically for my fork model (which is a Kona Project 2 440mm 1 1/8th ")? Thanks.

There can be differences, some are the same but with different markings. You need to order it by headset rather than by bike. What happened to the old one? If you're upgrading forks, it will come off the old ones.

Surely as long as you get the right size, it's the headset that decides?

It needs to match your headset.

Yes it has to match the headset, not all bearings are the same, and it has to match the lower bearing.

No, there's lots of different ones. The taper(s) on the race need to match the taper(s) on the bottom cup else you will end up with limited contact points between the cup and race. At best you won't be able to get the headset loaded tightly. At worst, no idea.