Hello and welcome, lots of info in the getting started section, the vetted sellers list is a good place to start.
I have ATA armor and on to my 2nd set after 7 years of trooping, my first set was just a bit scuffed from trooping so is now converted to a Sandtrooper.
Also have a look through some of the build sections, will give you a good idea of some differences between the armor.
Hope to see a build thread from you in the not too distant future.
Good luck with the research.