If you're thinking about carbon arrows then most measure around .295" which equates to a point diameter of 19/64". The two most common sizes used on them are 9/32" (a hari small) or 5/16" (a hair big) because that is what is most commonly sold. It's a pretty rare shop that carries 19/64" field tips.
I don't worry about wasting time looking for 19/64's field points. Main thing I do is shoot GoldTips. For their 19/64's dia shafts, the inserts are flared out to take a 5/16's point. For other carbons, I chuck up on the point's threads with a handdrill to spin it (I tell my wife it's my lap lathe
) and
carefully use a file to chamfer the shoulder of the field point so that it matches up with the insert. It only takes a tiny bit of weight off the point, not nearly enough to notice any difference in spine or shooting qualities.