Mike,
That needle will only work if you have a large tip also.
There are several places to buy the needles online including Dixie Art, BearAir, etc.
In regards to being confused by the parts, the needles are numbered 50-0401, 50-0402, and 50-0403. Those are fine, medium, and large needles respectively. So you will want the 50-0402 for a spare medium needle.
Mike
“Wisdom is the right use of knowledge. To know is not
to be wise. Many men know a great deal, and are all the greater fools
for it. There is no fool so great a fool as a knowing fool. But to know
how to use knowledge is to have wisdom. " Charles Spurgeon