Re: 75gr Hornady HPBT out of a 22-250
I am surprised to hear you don't like ADI powders - why not? ADI powder is probably the most widely used powder in the world right now, so not sure what you are finding lacking.
Alliant and Vihtavuori powders are both available in Australia through Noia, so you could always see if Noia can direct you to a dealer near by that caries these powders. But with a few very specific cases, I don't believe these powders offer much over the ADI powders.
As for velocities, the only way to know is to chronograph your rifle, because anything else is a very indicative indication. However I don't bother much with velocity until I have the load right on paper - then I just need to know velocity for the ballistics table.